Introduce class_names helper

Render objects that respond_to render_in in controllers

Fix docs about etag

In https://github.com/rails/rails/pull/36388,
we supported passing objects that `respond_to` `render_in`
to `render`, but _only_ in views.

This change does the same for controllers, as Rails
generally gives the expectation that `render` behaves
the same in both contexts.
Co-authored-by: NAaron Patterson <tenderlove@github.com>

As a follow-up to https://github.com/rails/rails/pull/37872,
this change introduces a class_names view helper
to make it easier to conditionally apply class names
in views.

Before:
<div class="<%= item.for_sale? ? 'active' : '' %>">

After:
<div class="<%= class_names(active: item.for_sale?) %>">

We've been using this helper in the GitHub monolith
since 2016.
Co-authored-by: NAaron Patterson <tenderlove@github.com>

Database URL supports query value with equal sign

Fix input value not properly applied:

- #37872 introduced a regression and you can't do

  ```html.erb
    hidden_field_tag('token', value: [1, 2, 3])
  ```

  This will result in a `<input type="hidden" value=""`>.

  I chose `hidden_field_tag` and the `value` attribute as an example
  but this issue applies to any tag helper and any attributes.

  https://github.com/rails/rails/pull/37872#issuecomment-561806468
  mention that the feature should only apply for "class" attribute.

  This commit fix original intent of #37872

Retain selections with `includes` and `joins`

A querystring value should be allowed to include an equal sign `=`.

This is necessary to support passing `options` for a PostgresSQL connection.

```

development:
  url: postgresql://localhost/railsdevapp_development?options=-cmysetting.debug=on
```

Before this PR, attempting to start the rails process with that configuration would result in an error:

```
> bundle exec rails console
Traceback (most recent call last):
	49: from bin/rails:4:in `<main>'
	48: from bin/rails:4:in `require'
...
	 1: from /rails/activerecord/lib/active_record/database_configurations/connection_url_resolver.rb:58:in `query_hash'
/rails/activerecord/lib/active_record/database_configurations/connection_url_resolver.rb:58:in `[]': invalid number of elements (3 for 1..2) (ArgumentError)
```

After this PR, rails can properly parse the configuration:

```
> bundle exec rails console
Loading development environment (Rails 6.1.0.alpha)
2.6.5 :001 > ActiveRecord::Base.connection.select_all("show mysetting.debug").to_a
   (0.4ms)  show mysetting.debug
 => [{"mysetting.debug"=>"on"}]
```

Added posibility to open a `read_uncommitted` transaction on SQLite:

Don't run AJ after_enqueue / after_perform when chain is halted:

- ### Problem

  ```ruby
    MyJob < ApplicationJob
      before_enqueue { throw(:abort) }
      after_enqueue { # enters here }
    end
  ```
  I find AJ behaviour on after_enqueue and after_perform callbacks
  weird as they get run even when the callback chain is halted.
  It's counter intuitive to run the after_enqueue callbacks even
  though the job wasn't event enqueued.

  ### Solution

  In Rails 6.2, I propose to make the new behaviour the default
  and stop running after callbacks when the chain is halted.
  For application that wants this behaviour now or in 6.1
  they can do so by adding the `config.active_job.skip_after_callbacks_if_terminated = true`
  in their configuration file.

Fix ActiveJob logging when callback chain is halted:

- ### Problem

  ActiveJob will always log "Enqueued MyJob (Job ID) ..." even
  if the job doesn't get enqueued through the adapter.
  Same problem happens when performing a Job, "Performed MyJob (Job ID) ..." will be logged even when job wasn't performed at all.
  This situation can happen either if the callback chain is terminated
  (before_enqueue throwing an `abort`) or if an exception is raised.

  ### Solution

  Check if the callback chain is aborted/exception is raised, and  log accordingly.

- SQlite allow database to be specified as URL (given that URI filename
  interpretation was turned on on the connection.)

  This commit is necessary for the read_uncommitted transaction
  feature because SQLite doesn't allow to enable the shared-cache mode
  if the database name is `:memory:`. It has to be a URI (`file::memory`)

  Ref https://www.sqlite.org/sharedcache.html#shared_cache_and_in_memory_databases

- ### Use case

  I'd like to be able to see changes made by a connection writer
  within a connection reader before the writer transaction commits
  (aka `read_uncommitted` transaction isolation level).

  ```ruby
  conn1.transaction do
    Dog.create(name: 'Fido')
    conn2.transaction do
      Dog.find(name: 'Fido') # -> Can't see the dog untill conn1 commits the transaction
    end
  end
  ```

  Other adapters in Rails (mysql, postgres) already supports multiple
  types of isolated db transaction.
  SQLite doesn't support the 4 main ones but it supports
  `read_uncommitted` and `serializable` (the default one when opening
  a transaction)

  ### Solution

  This PR allow developers to open a `read_uncommitted` transaction by
  setting the PRAGMA `read_uncommitted` to true for the duration
  of the transaction. That PRAGMA can only be enabled if the SQLite
  connection was established with the [shared-cache mode](https://www.sqlite.org/sharedcache.html)

  This feature can also benefit the framework and we could potentially
  get rid of the `setup_shared_connection_pool` inside tests which
  was a solution in the context of a multi-db app so that the reader
  can see stuff from the open transaction writer but has some [caveats](https://github.com/rails/rails/issues/37765#event-2828609021).

  ### Edge case

  Shared-cache mode can be enabled for in memory database as well,
  however for backward compatibility reasons, SQLite only allows
  to set the shared-cache mode if the database name is a URI.
  It won't allow it if the database name is `:memory`; it has to be
  changed to `file::memory` instead.

This reverts commit 4e105385, reversing
changes made to 62b43839.

The change in Ruby that made those changes required was reverted in
https://bugs.ruby-lang.org/projects/ruby-trunk/repository/git/revisions/8852fa876039ed177fd5e867f36177d8a9ff411c

Rails::Application#config_for merges shared configuration deeply

[ci skip]

Add a yellow "NOTE" block to see if the guide looks better.

Remove unused query params in DiskService

Use DiskController for both public and private files

Bring back feature that allows loading external route files: