View files is necessary to send mail, it should be generated even Rails API.
Fixes #23617

3f70e8bd set a new bar for tests, but they weren't updated.

Update their expectation.

It has been changed to require `belongs_to` by default in Rails 5.
Therefore in order to pass the controller test, have association of set to fixtures.

Fixes #23384

- app generate option --skip-sprockets leaves jquery-rails gem, which relies on sprockets environment
- Remove jquery-rails if --skip-sprockets is true

Fixes #23431

Redis now included in Gemfile but commented out. This change was made in
91864439 and is causing the test
failure.

See https://travis-ci.org/rails/rails/jobs/106994913#L1025

Test of Rails API should be in `api_app_generator_test.rb`.

Generated engines should call `protect_from_forgery`.  If this method
isn't called, then the Engine could be susceptible to XSS attacks.
Thanks @tomekr for reporting this to us!

This new adapter does get a little more intimate with the redis-rb gem's
implementation than I would like, but it's the least bad of the
approaches I've come up with.

When the `puma` command is run without any configuration options it will detect presence of a `config/puma.rb` file and use that. Currently there is discrepancy between `puma` command and `rails server` but Evan said it would be reasonable to add in reading in config from the default location. I am working on that right now as a feature in puma/puma.

Why do we need this? By default Puma uses 16 threads, and by default ActiveRecord only has 5 threads. Due to the architecture of AR it is guaranteed that if you're running with fewer DB connections than your server has threads you will hit `ActiveRecord::ConnectionTimeoutError ` eventually if your app gets modest amounts of traffic. Since we are providing a default webserver, we should provide reasonable configuration for that webserver.

This PR does a few things, first it sets the default Puma thread count to 5 to mach ActiveRecord's default. It sets the default environment to `"development"` and the default port to 300 so that booting the server with `$ puma` will give you the same default port as `rails server`. It is worth mentioning that by reading in from `PORT` environment variable this config can work with containerized deployments, such as on Heroku. 

We are not using worker processes by default, that way JRuby and windows devs can use this configuration without modification. I went ahead and included a default `on_worker_boot`. It won't be used unless a worker count is specified, that means this config will not use it. Even though it's not being used now It will make someone who wants to try modifying their config to run extra workers easier.

cc/ @pixeltrix

- Using `references` or `belongs_to` in migrations will always add index
  for the referenced column by default, without adding `index:true` option
  to generated migration file.
- Users can opt out of this by passing `index: false`.
- Legacy migrations won't be affected by this change. They will continue
  to run as they were before.
- Fixes #18146

- Fixes #23137.

Because the form is not in the Rails API,
`per_form_csrf_tokens` initializer I think unnecessary.

Forgot to remove it, when I changed the expectations in 88881d2b.

since 9446e38b, generated mailer inherents from ApplicationMailer,
ApplicationMailer is required in the mountable engine.

It was removed by mistake at 877a411d

`rack-cors` gem is defined in Gemfile by default only if the api,
not defined by default in rails app.

The Action Cable generators creates four files which need to be removed
if `skip_action_cable` is set.

1. `app/assets/javascripts/cable.coffee`
2. `app/channels/application_cable/channel.rb`
3. `app/channels/application_cable/connection.rb`
4. `config/redis/cable.yml`

Fixes #22669.

generator template.

The errors message only was not displayed, as if it did not use the inline reporting,
modified to also information the method name and the like in error are displayed.

```
# before
Failed assertion, no message given.

bin/rails test test/models/user_test.rb:5
```

```
# after
Failure:
UserTest#test_the_truth:
Failed assertion, no message given.

bin/rails test test/models/user_test.rb:5
```

Our logic is complex now and we don't need to check the version to asset
the behavior so I'm removing the checking here.

We originally chose to apply very strict versioning on the `rails` entry
in the Gemfile, because our future versioning policy was not strongly
defined.

Now it is, and our policy is very much designed on the expectation that
people will regularly update to the latest patch level in their release
series... so we should encourage that.

Of course, Gemfile.lock will do its job and prevent unplanned updates,
just as it does for every other gem in the bundle... but if you run
`bundle update`, we want to get you the latest bug/security fixes
without requiring a manual edit of the Gemfile entry.

Our current version could be a few different shapes, so it takes a bit
of work to find the right specifier, but in principle, we match anything
of the form x.y.*, where x.y matches our current release series.

I think Markdown is nowadays a better default.