- Test was wrongly using assert with assignment statement.

Fixes #18473

For mailers created via generators

Follow up to #18074

Following the same naming convention used in
controllers and jobs.

bebugger doesn't work with Ruby 2.2 so we don't need to support it
anymore

`silence_stderr`, `silence_stream`, `capture` and `quietly`.

Related to #13434.

Fix #18301

This stems from [a comment](rails#17227 (comment)) by @dhh.
In summary:

* New Rails 5.0 apps will not accept `return false` as a way to halt callback chains, and will not display a deprecation warning.
* Existing apps ported to Rails 5.0 will still accept `return false` as a way to halt callback chains, albeit with a deprecation warning.

For this purpose, this commit introduces a Rails configuration option:

```ruby
config.active_support.halt_callback_chains_on_return_false
```

For new Rails 5.0 apps, this option will be set to `false` by a new initializer
`config/initializers/callback_terminator.rb`:

```ruby
Rails.application.config.active_support.halt_callback_chains_on_return_false = false
```

For existing apps ported to Rails 5.0, the initializers above will not exist.
Even running `rake rails:update` will not create this initializer.

Since the default value of `halt_callback_chains_on_return_false` is set to
`true`, these apps will still accept `return true` as a way to halt callback
chains, displaying a deprecation warning.

Developers will be able to switch to the new behavior (and stop the warning)
by manually adding the line above to their `config/application.rb`.

A gist with the suggested release notes to add to Rails 5.0 after this
commit is available at https://gist.github.com/claudiob/614c59409fb7d11f2931

This commit modifies the code (but not the purpose) of a test that checks that

> initializers are executed after application configuration initializers

Currently the test hard-codes the *exact* initializers that are expected to
occur before a custom one. This can cause the test to fail even if the
expectation still passes.

This commit loosens the test by simply checking that, in the array of
initializers, the custom initializers (called `dummy_initializer` in the
example) is executed after the last occurrence of `load_config_initializers`.

    warning: ambiguous first argument; put parentheses or a space even after
    `/' operator'`

Thanks @robin850.

Changes `rails g model Post user:references` from

    def change
      create_table :posts do |t|
        t.references :user, index: true
      end

      add_foreign_key :posts, :users
    end

to

    def change
      create_table :posts do |t|
        t.references :user, index: true, foreign_key: true
      end
    end

Changes `rails g migration add_user_to_posts user:references` from

    def change
      add_reference :posts, :users, index: true
      add_foreign_key :posts, :users
    end

to

    def change
      add_reference :posts, :users, index: true, foreign_key: true
    end

The changes in #18149 added tests for the app generator, but only fixed
it for the plugin generator (I should have let CI finish though I think
it would have failed as an allowed failure).

Allow static asset serving from env variable (enhanced!)
Conflicts:
	railties/CHANGELOG.md

Closes #17945

`db:test:prepare` still purges the database to always keep the test
database in a consistent state.

This patch introduces new problems with `db:schema:load`. Prior
to the introduction of foreign-keys, we could run this file against
a non-empty database. Since every `create_table` containted the
`force: true` option, this would recreate tables when loading the schema.

However with foreign-keys in place, `force: true` wont work anymore and
the task will crash.

/cc @schneems

Fixes https://github.com/rails/rails/issues/17340.

Use Minitest::BacktraceFilter instead of removing all silencers. This
will allow the backtrace for all libraries in the plugin to be shown
while removing noise generated by Minitest's backtrace.

See https://github.com/rails/rails/commit/9b15828b5c347395b42066a588c88e5eb4e72279#commitcomment-8764492