This patch removes the tasks doc:app, doc:rails, and doc:guides.

In our experience applications do not generate APIs using doc:app.
Methods may be certainly documented for maintainers, annotated
with YARD tags, etc. but that is intended to be read with the
source code, not in a separate website. Then, teams also have
typically selected topics written down in Markdown files, or in
a GitHub wiki... that kind of thing.

If a team absolutely needs to generate application documentation
for internal purposes, they can still easily write their own task.

Regarding doc:rails and doc:guides, we live in 2015. We are used
to go to online docs all the time. If you really want access to the
API offline RubyGems generates it for every Rails component unless
you tell it not to, and you can checkout the Rails source code to
read the guides as Markdown, or download them for a Kindle reader.

All in all, maintaining this code does not seem to be worthwhile
anymore.

As a consequence of this, guides (+3 MB uncompressed) won't be
distributed with the rails gem anymore. Of course, guides and API
are going to be still part of releases, since documentation is
maintained alongside code and tests.

Also, time permitting, this will allow us to experiment with novel
ways to generate documentation in the Rails docs server, since
right now we were constrained by being able to generate them in
the user's environment.

Thor isn't very discerning over whether some content is present when passed
to `inject_into_file`, e.g. a commented out route is detected as being present.
So to prevent people scratching their heads as to why a route hasn't appeared
it's better to fall on the side of having duplicate routes.

When there is a single scaffold in the routes.rb with no other lines
then revoking/destroying it will create a routes.rb file with a syntax
error. This is because the sentinel for the Thor `route` action didn't
include the newline but the logged route code did.

The fix is to add the newline to the sentinel and remove it from the
the logged route code.

Fixes #15913.

Mostly cosmetics, except that `isolate_namespace` should be applied to the last
module in the chain.

It was not possible to create a new gem with a hyphenated name via the `rails plugin new` generator.

The naming guide of rubygems clearly says dashes should be used for gems that extend other gems. http://guides.rubygems.org/name-your-gem/

It's a thin layer to provide easy access to sample files throughout
test-cases. This adds the directory `test/fixtures/files` to newly
generated applications.

Rails no longer generates Test::Unit files by default.

- Added include for the same in ActiveSupport::Test.
- Removed occurrences of silence_stream being used elsewhere.
- Reordered activesupport testcase requires alphabetically.

- Removed require of silence stream from test_case

- Moved quietly method to stream helper

- Moved capture output to stream helper module and setup requires for the same elsewhere

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

    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).

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.

If you run a generator such as:

```
rails generate model accounts supplier:references
```

The resulting migration will now add the corresponding foreign key
constraint unless the reference was specified to be polymorphic.

Creates an ApplicationMailer and layout by default, including html and body tags to reduce spam score