Add process.action_mailer notification to Instrument guide [ci skip]

Bump RuboCop 0.50.0

Yield with an error instance instead of error class

RuboCop 0.50.0 was released.
https://github.com/bbatsov/rubocop/releases/tag/v0.50.0

And `rubocop-0-50` channel is available in Code Climate.
https://github.com/codeclimate/codeclimate-rubocop/issues/107#issuecomment-336234260

This commit will bump RuboCop to 0.50.0.
There are no new offences in this change.

```console
% bundle exec rubocop --version
0.50.0
% bundle exec rubocop
Inspecting 2350 files

(snip)

2350 files inspected, no offenses detected
```

fix the description for the `select_all` [ci skip]

[ci skip]Enable link to ActionDispatch::Integration::Session#process in rdoc

Fix "warning: assigned but unused variable - message"

Ruby 2.5 warns about this. Ref: https://travis-ci.org/rails/rails/jobs/286338999

Fix some typos [ci skip]

Add allow_other_host option to redirect_back method

Update security guide for signed cookie rotations

The example was slightly incorrect. This commit also adds a test case
for this example to cookies middleware unit tests.

Improve railties' tests

Include Mutex_m into GeneratedAttributeMethods instead of extending instance

Simplify parse arguments in `ConsoleTest`

Express #change_column_comment as public api

Use released `sidekiq` instead of master version

The sidekiq 5.0.5 includes redis-rb 4.0 support.
Ref: https://github.com/mperham/sidekiq/blob/90db3b84208cbb73a50d1a77a1dea97d3490ce70/Changes.md#505

Building association scope in join dependency should respect the parent
relation's aliases to avoid using the same alias name more than once.

Fixes #30681.

Use `form_with` instead of `form_for` in engine guide [ci skip]

I don't think this is a good abstraction because the internal method is
used only if the relation need to be applied join dependency.

Fix the result of `rails routes` in Guide [ci skip]

`relation.exists?` should reference correct aliases while joining tables
of has_many through associations.

Seems the branch has rebased. cc @jeremy

Fix typo in loading error message

s/Action Record/Active Record/