Instructions to use `h` or `html_escape` in ERB templates were added to 
`actionpack/lib/action_view/template_handlers/erb.rb` in a1b03493 (Rails 
2.1), but ERB has automatically escaped values since Rails 3.

`:api:` tag was removed in 5349f231 since RDoc doesn't support `:api:`
tag. But those methods are not private API, they are public API for
renderers. The renderers should be able to know that they can override
this method.

PhantomJS is abandoned, replace it with Selenium/Chrome headless

Fix callback in rails ujs

Remove unused `FixtureTemplate` class

`FixtureTemplate` is no longer used since 3d7892d4.

Fix `bin/rails db:setup` and `bin/rails db:test:prepare` create  wrong ar_internal_metadata's data for a test database.

Improve out-of-box experience with System tests including chromedriver-helper by default

Enable `hstore` extention disabled at the end of `test_migrate_enable_and_disable_extension`

Enable `hstore` extention disabled at the end of `InvertibleMigrationTest#test_migrate_enable_and_disable_extension`
to avoid failure of `PostgresqlArrayTest#test_schema_dump_with_shorthand`
which expects `hstore` extension enabled.

Set proper migration' version in bug report templates

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.

  When load schema from `structure.sql`, database connection isn't
  established. `ActiveRecord::Tasks::DatabaseTasks.load_schema` has to
  establish database connection since it executes
  ```
  ActiveRecord::InternalMetadata.create_table
  ActiveRecord::InternalMetadata[:environment] = environment
  ```

Fix `bin/rails db:setup` and `bin/rails db:test:prepare` create  wrong ar_internal_metadata's data for a test database.

  Before:
  ```
  $ RAILS_ENV=test rails dbconsole
  > SELECT * FROM ar_internal_metadata;
  key|value|created_at|updated_at
  environment|development|2017-09-11 23:14:10.815679|2017-09-11 23:14:10.815679
  ```

  After:
  ```
  $ RAILS_ENV=test rails dbconsole
  > SELECT * FROM ar_internal_metadata;
  key|value|created_at|updated_at
  environment|test|2017-09-11 23:14:10.815679|2017-09-11 23:14:10.815679
  ```

  Fixes #26731.

Follow up of #30862.

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

Capitalize "sprockets" in Engine guide [ci skip]