Configure Active Storage route prefix

* Update association_basics.md [ci skip]

Addresses issue #33599

* Update associations.rb API documentation [ci skip]

Additional detail for documentation of `belongs_to` `association=(associate)`

* Update association_basics.md [ci skip]

Reverts misplaced documentation comment

[ci skip] Fix documentation for Response#content_type

Applications can configure the route prefix prepended to the Active
Storage routes. By default this maintains the previous prefix
`/rails/active_storage` but supports custom prefixes.

Before this change the route for serving blobs is fixed to
`/rails/active_storage/blobs/:signed_id/*filename`. After this change
it's possible to configure the route to something like
`/files/blobs/:signed_id/*filename`.

Remove mysql2 gem version requirement "< 0.6.0"

Follow up #33874.
Related #23393.

Suggested at https://github.com/rails/rails/pull/33876#issuecomment-421176221

Currently, PostgreSQL + Ruby 2.5.1 build has failed.

https://travis-ci.org/rails/rails/jobs/427502723#L2940-L2942
https://travis-ci.org/rails/rails/jobs/427496382#L2921-L2923
https://travis-ci.org/rails/rails/jobs/427470858#L2921-L2923

This is because PostgreSQL server abnormally terminates when ramfs
becomes full.
Ref: https://travis-ci.org/rails/rails/jobs/427960998#L7614

This occurs since d54d0c95.
In d54d0c95, `sudo` is used in preprocessing
for setting MySQL.

In Travis, even if specified `sudo: false`, when using `sudo` in build,
sudo-enabled infrastructure is used.
https://docs.travis-ci.com/user/reference/overview/#virtualisation-environment-vs-operating-system

As container-based infrastructure was changed to sudo-enabled infrastructure,
the disk settings etc were also changed, it seems that this phenomenon occurred.

This can also be handled by extending ramfs(https://github.com/travis-ci/travis-ci/issues/9036#issuecomment-357189213).
However, using sudo-enabled in all builds affects the execution time of
build, so I think that it is good to use sudo-enabled only in the necessary builds.

SQLite3 adapter supports expression indexes

Bump MySQL version requirement to 5.5.8 in the template files

[ci skip]

Improve API document on Object#blank?

Follow-up #33853

Conditionally use `helper_method` in Flash concern

add `any?` to DatabaseConfigurations

Allow subclasses to redefine autosave callbacks for associated records

 Validate if `utf8mb4` character set and longer index key prefix is supported

Update arguments.rb [ci skip]

Fix typo `Instrinsic` -> `Intrinsic`

Add changelog entries for #33849 [ci skip]

Since these changes related to the public API, I think we should add
changelog entries.

Related to #33838, #33849

Eager loading/preloading should be worked regardless of large number of records

* MySQL 5.1 does not support `utf8mb4` character set

* MySQL 5.1 had been already EOLed on Dec 2013
https://www.mysql.com/support/eol-notice.html

> Per Oracle's Lifetime Support policy, as of December 31, 2013, MySQL 5.1
> is covered under Oracle Sustaining Support.

* MySQL 5.5.8 is the first General Availability of MySQL 5.5

https://dev.mysql.com/doc/relnotes/mysql/5.5/en/news-5-5-8.html

Once #33608 merged If users create a new database using MySQL 5.1.x, it will fail to create databases
since MySQL 5.1 does not know `utf8mb4` character set.

This pull request removes `encoding: utf8mb4` from `mysql.yml.tt`
to let create_database method handles default character set by MySQL server version.

`supports_longer_index_key_prefix?` method will need to validate if MySQL 5.5 and 5.6 server configured
correctly to support longer index key prefix, but not yet.

Use faster globs for template resolving

In production the query cache was already being loaded before the first
request even without #33856, so added a test to make sure of it.

This new test is passing even if #33856 is reverted.

Include test helpers in ActionDispatch::IntegrationTest

As @dhh brings up, the point of `ActionDispatch::IntegrationTest` is to
allow users to test the integration of all the pieces called by a
controller. Asserting about the emails and jobs queued is part of that
task.

This commit includes the `ActionMailer::TestHelper` and
`ActiveJob::TestHelper` modules when the ActionMailer and ActiveJob
railties are initialized respectively.

Faster multiple_of? method

Follow up #33756

Also remove `# :nodoc:` for `ActiveRecord::Core::ClassMethods` in order
to show non-nodoc methods in that module on the api docs http://edgeapi.rubyonrails.org