Remove the word "mongrel" from documents [ci skip]

Currently mongrel is not maintained.
And it couldn't be built with any Ruby versions that
supported by Rails.

It is reasonable to remove the word "mongrel" in order to avoid
confusion from newcomer.

Fix Remaining Case-In-Assignment Statement Formatting

Minor grammar fix in engines.md [ci skip]

Recently, the Rails team made an effort to keep the source code consistent, using Ruboco
(bb1ecdcc and below). Some of the case
statements were missed.

This changes the case statements' formatting and is consistent with changes
in 810dff7c and db63406c.

Remove redundant `!loaded?` condition

Remove not used alias method

Bang methods of `AR::QueryMethods` are used only internally.
We only use `left_outer_joins!`, so we can remove this alias.

Already checked `if !find_target? || loaded?`, unnecessary `!loaded?` in
elsif condition.

Move `require "models/post"` before `require "models/comment"`

use `descendants` to get class that inherited `ActiveJob::Base`

Remove unnecessary `query_scope`

change `Class#descendants` to public API [ci skip]

`Class#descendants` has already been displayed in Rails guide,
so I think that may be displayed in doc.
http://guides.rubyonrails.org/active_support_core_extensions.html#descendants

`subclasses` get only child classes.
Therefore, if create a job common parent class as `ApplicationJob`,
inherited class does not get properly.

rename test method to avoid overriding

Remove unnecessary `count` method for collection proxy

make `fixture_file_upload` work in integration tests

Currently, `fixture_file_upload` does not work in integration test.
Because, `TestProcess` module has been include in `Session` class, but
`fixture_path` can not get from `Session` class.

Modify to include `TestProcess` in `IntegrationTest` class in order to get
correct value of `fixture_path`.

Simply use its own method because `CollectionProxy` inherits `Relation`.

Update ActiveRecord callback guide for exceptions [ci skip]

07d3d402 changed how exceptions are handled in after_commit
and after_destroy callbacks.  This commit updates the 5.0
release notes and the  ActiveRecord callback guide to reflect
the new behavior.

[ci skip]

Allow download of email attachments in the Mailer Preview template.

Remove blank else branch

Extract duplicated `create` and `create!` definition for association

Fix broken heredoc indentation caused by rubocop auto-correct

This removes the following warning.

```
./test/dispatch/routing_test.rb:3696: warning: method redefined; discarding old test_namespaced_roots
./test/dispatch/routing_test.rb:1632: warning: previous definition of test_namespaced_roots was here
```

All indentation was normalized by rubocop auto-correct at 80e66cc4.
But heredocs was still kept absolute position. This commit aligns
heredocs indentation for consistency.

add Active Job bug report templates to guide [ci skip]

Follow up to #26362

to address BasicsTest#test_readonly_attributes failure #26368

It reproduces only when both of these conditions are satisfied:

- Other test files `autosave_association_test.rb` or `where_test.rb`
  which executes `require "models/comment"` then `require "models/post"`
- When `autosave_association_test.rb` or `where_test.rb` executed before `base_test.rb`

fix class name typo in bug report templates for Active Job

Improve Getting Started [ci skip]