Add mention how to delete several records in bulk to Active Record Basics Guides [ci skip]

Remove unused `UnknownController` class

`UnknownController` was added in b1999be5, but it is not used anywhere.

Remove unnecessary `allow_failure: true` option

`routes` task always returns zero to status, so status is not to non-zeno.
Ref: https://github.com/rails/rails/blob/b1867c480dd5476948ff0492ea2f91e2c2fcb04b/railties/lib/rails/tasks/routes.rake#L30

Fix typo in `allow_failure` argument [ci skip]

Ref: https://github.com/rails/rails/blob/3be123ba26cad461a80d7d680819e71c1388a241/railties/test/isolation/abstract_unit.rb#L243

42 compatibility - use int instead of bigint

Add headless chrome driver to System Tests

Update mailing list URL in I18n guide [ci skip]

Fix `to_s(:db)` for range comprising of alphabets.

Remove association(true) references from docs [ci skip]

Passing `true` to force an association to reload its records from the
database was deprecated in 5.0 and removed in 5.1.

Follow up of #30862.

Remove a needless space in Action View Guide [ci skip]

Capitalize "sprockets" in Engine guide [ci skip]

This test covers the case of 02da8aea.

Previously `exists?` was always eager-loading the includes values. But
now it is eager-loaded only when necessary since 07a611e0.

So the case of the eager-loading had not covered in the test.

We already found the longer sequence name, but we could not consider
whether it was the sequence name created by serial type due to missed a
max identifier length limitation. I've addressed the sequence name
consideration to respect the max identifier length.

Fixes #28332.

Update Gemfile with comment for Active Storage group

Currently `AUTO_INCREMENT` is implicitly used in the default primary key
definition. But `AUTO_INCREMENT` is not only used for single column
primary key, but also for composite primary key. In that case,
`auto_increment: true` should be dumped explicitly in the
`db/schema.rb`.

Fixes #30894.

Remove unused `before_filters`

This method added by 10085114. It is unnecessary because it is no longer called
by 19c3495a.

Add accept-charset to the output of form_with in JS guide [ci skip]

This is the fix for the regression of #29848.

In #29848, I've kept existing select list in the subquery for the count
if ORDER BY is given. But it had accidentally affect to GROUP BY
queries also. It should keep the previous behavior in that case.

Fixes #30886.

For investigating the cause of failure.

https://travis-ci.org/rails/rails/jobs/287474883#L797-L799

Add unpermitted_parameters hook to instrumentation guide [ci skip]

Fix test name for daemon option test

In this test file, "server option" refers to the server used to start
Rails(e.g. `puma`, `thin`).
But this test, "server option" is not specified. Therefore, I think that
it is incorrect that `server_option` is included in the test name.

Remove unsued `jquery-rails`

Use __dir__  in activestorage/

Added test case for starting rails with daemon option

Add Action Cable notifications to instrument guide [ci skip]