Using `undef_method` means that when a route is removed any other
implementations of that method in the ancestor chain are inaccessible
so instead use `remove_method` which restores access to the ancestor.

Allow the use of `direct` to specify custom mappings for polymorphic_url, e.g:

  resource :basket
  direct(class: "Basket") { [:basket] }

This will then generate the following:

  >> link_to "Basket", @basket
  => <a href="/basket">Basket</a>

More importantly it will generate the correct url when used with `form_for`.

Fixes #1769.

Allow the definition of custom url helpers that will be available
automatically wherever standard url helpers are available. The
current solution is to create helper methods in ApplicationHelper
or some other helper module and this isn't a great solution since
the url helper module can be called directly or included in another
class which doesn't include the normal helper modules.

Reference #22512.

The singleton url_for on Rails.application.routes.url_helpers isn't the
same as the url_for you get when you include the module in your class as
the latter has support for polymorphic style routes, etc. whereas the
former accepts only a hash and is the underlying implementation defined
on ActionDispatch::Routing::RouteSet.

This commit changes the singleton method to call through a proxy instance
so that it gets the full range of features specified in the documentation
for url_for.

Fix `define_attribute_method` with Symbol in AR

This issue is only appear when you try to call `define_attribute_method`
and passing a symbol in Active Record. It does not appear in isolation
in Active Model itself.

Before this patch, when you run `User.define_attribute_method :foo`, you
will get:

    NoMethodError: undefined method `unpack' for :foo:Symbol
        from activerecord/lib/active_record/attribute_methods/read.rb:28:in `define_method_attribute'
        from activerecord/lib/active_record/attribute_methods/primary_key.rb:61:in `define_method_attribute'
        from activemodel/lib/active_model/attribute_methods.rb:292:in `block in define_attribute_method'
        from activemodel/lib/active_model/attribute_methods.rb:285:in `each'
        from activemodel/lib/active_model/attribute_methods.rb:285:in `define_attribute_method'

This patch contains both a fix in Active Model and a test in Active
Record for this error.

Do not display template files on API doc [ci skip]

```
Minitest.run_via[:rails] = true
```

👆 would break because a simple alias won't catch the second
true argument there.

Use Puma 3.7

Fix some grammar in docs [ci skip]

Use `response#location` instead of `#location` in redirect.

ref this commit seems that has not been merged into 3.7 https://github.com/puma/puma/commit/42bec4600c51ab8a1c1ee5a0e1b738a4ffd82bf2

Ensure test threads share a DB connection

[ci skip]

WIP: Capybara Integration with Rails (AKA System Tests)

This ensures multiple threads inside a transactional test to see consistent
database state.

When a system test starts Puma spins up one thread and Capybara spins up
another thread. Because of this when tests are run the database cannot
see what was inserted into the database on teardown. This is because
there are two threads using two different connections.

This change uses the statement cache to lock the threads to using a
single connection ID instead of each not being able to see each other.
This code only runs in the fixture setup and teardown so it does not
affect real production databases.

When a transaction is opened we set `lock_thread` to `Thread.current` so
we can keep track of which connection the thread is using. When we
rollback the transaction we unlock the thread and then there will be no
left-over data in the database because the transaction will roll back
the correct connections.

[ Eileen M. Uchitelle, Matthew Draper ]

Prevent multiple values being set to `run_via`

Pointed out by @matthewd that the HWIA subclass changes the
AS scoped class and top-level HWIA hierarchies out from under
existing classes.

This reverts commit 71da3909, reversing
changes made to 41c33bd4.

There were some grammar issues and incorrect information in the system
tests documentation.

This renames the system test helper file to be application system test
case to match what the rest of Rails does. In the future we should
consider changing the test_helper to match.

* Override integration test default host

Integration tests automatically set the default host to
'http://example.com'. This works fine for integration tests because they
are not real browser sessions, but doesn't work fine for system tests
because they are real browser sessions.

We can override this by setting the `host!` in `before_setup. The
`Capybara.always_include_port` will allow the test to look at
`127.0.0.1:port capybara picks` and properly redirect the test.

Any application can override this by setting the `host!` in
their system test helper. Generally though, applications are going to be
using localhost.

In this commit I also moved the setup and teardown into their own module
for tidiness.

* Move teardown settings into system test case

These configuration options can be put into the system test case file
instead of the generated system tests helper file. This is an
implementation detail and therefore shouldn't be generated with the
template.

We only want the file name to include the word `failures` if it failed,
not any time the user wants to take a screenshot during a test run.

This serves as self documentation so users know how to change the
driver.

* Move system tests back into Action Pack
* Rename `ActionSystemTest` to `ActionDispatch::SystemTestCase`
* Remove private base module and only make file for public
`SystemTestCase` class, name private module `SystemTesting`
* Rename `ActionSystemTestCase` to `ApplicationSystemTestCase`
* Update corresponding documentation and guides
* Delete old `ActionSystemTest` files

I've renamed the server to `rails_puma` so that it doesn't override
Capybara's default puma server. I've also removed the hard port setting.
Users can simply use `Capybara.server_port` writer to set the port if
they require that.

Update the documentation after rewriting a majority of the functionality
for system testing.

This is a major rewrite of what existed previously. After discussing
this feature with DHH I realized that I was looking at the setup all
wrong.

I had originally mentally broken it into "what Rails wants" and "what
Capybara already has".

What happened after looking at it from DHH's angle was that I saw there
was no reason to group settings by Driver but instead the following
groups:

- There will always be a `Driver`
  - This can selenium, poltergeist, or capybara webkit. Capybara already
    provides all of these and there's no reason to break them into a
    category of Rails' usese Selenium like this and Capybara uses it
    like that.
- Only Selenium drivers care about `Browser`
  - Because of this it was weird to set it only in the Rails end.
  - Therefore only `Browser`, and not `Driver` cares about
    `screen_size`.
- Puma is the default `Server` in Rails
  - Therefore there's no reason to explictly support Webkit

Once I looked at it from this angle I was able to abstract all the
settings away from grouping the drivers with their options.

Now all the driver, server, and browser settings are abstracted away and
not part of the public facing API.

This means there's no requirement to initialize new classes to change
the default settings and the public options API is much smaller.

All of Rails preferred defaults are still there (selenium with port
21800 using the chrome browser with a screen size of 1400x1400) but
changing these no longer requires initializing a new class or
understanding which driver you're using underneath (rails defaults or
capybaras defaults respectively). Rails opinions are now simple defaults
instead of doing a them versus us setup with Drivers and explicit
options.

Changing the defaults is simple. Call `driven_by` with different
settings to change the defaults which will on their own initialize new
classes and change the default settings.

Use poltergeist with port 3000 for Puma

```
driven_by :poltergeist, on: 3000
```

Use selenium with the Chrome browser and a screen size of 800x800
```
driven_by :selenium, using: :firefox, screen_size: [ 800, 800 ]
```

The entire setup of how browser and drivers interact with each other are
abstracted away and the only required argument is the driver name.

This adds the required guides for how to write and use system tests in
your application.

1. Clean up screenshot helper

Updates documentation to be clearer and separates the concerns of saving
the image, setting the image path, and displaying the image.

2. Remove Rails provided assertions for selectors

This was moved upstream to Capybara and is no longer necessary to be
included in Rails

3. Remove form helper

The form helper is pretty specific to Basecamp's needs and may not be
helpful outside of Rails.