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/

redis-rb 4.0 support

Adapters bubble up gem version mismatches for their dependencies

* Use `gem 'redis', '~> 4.0'` for new app Gemfiles
* Loosen Action Cable redis-rb dep to `>= 3.3, < 5`
* Bump redis-namespace for looser Redis version dep
* Avoid using the underlying `redis.client` directly
* Use `Redis.new` instead of `Redis.connect`

* When the adapter is missing, raise an exception that points out config
  typos and missing Gemfile entries. (We can assume that a non-builtin
  adapter was used since these are always available.)
* When loading an adapter raises a LoadError, prefix its error message
  to indicate that the adapter is likely missing an optional dependency.

Fixes ActiveSupport::Cache::FileStore#cleanup bug which prevented it from removing expired entries

Break long variant filenames into path chunks

  - Remove redundant setting `RAILS_ENV` for `db:test:prepare`.
    `db:test:prepare` doesn't require it.

See #30520

Exception message for SystemTestCase#get etc..

This is preparation to respect parent relation's alias tracking for
fixing #30681.