Add missing require in ActionMailer::Base

Rails secrets is not built on but inspired by Sekrets

Put wording in line with http://weblog.rubyonrails.org/2017/4/27/Rails-5-1-final/

Without this, Action Mailer doesn't work like it used to on version 4.
The following snippet fails since version 5:

    > require 'action_mailer'
    > ActionMailer::Base
    NameError: uninitialized constant ActiveSupport::Rescuable

Deprecate `supports_statement_cache?`

Use `flat_map` rather than `map(&:...).flatten`

CLI arg `--port` has precedence over env `PORT`

Add tests for rendering with variants

Friendly maintain schema error in test runner

Update the Gem Bug Report Templates to 5.1.0

Remove unnecessary `package.json` deletion

5.1.0 has been released, and the gem templates can reflect that now.

The `package.json` is created only if `skip_yarn` is not specified.
https://github.com/rails/rails/blob/a4c1282854795d1f0d7696ce1ccbabf94b3d9098/railties/lib/rails/generators/rails/app/app_generator.rb#L202..L204

`supports_statement_cache?` was introduced in 3.1.0.beta1 (104d0b26) for
bind parameter substitution, but it is no longer used in 3.1.0.rc1
(73ff6797). Originally it should respect `prepared_statements` rather
than `supports_statement_cache?` (fd398475).
One more thing, named `supports_statement_cache?` is pretty misreading.
We have `StatementCache` and `StatementPool`. However,
`supports_statement_cache?` doesn't mean `StatementCache`, but
`StatementPool` unlike its name.

https://github.com/rails/rails/blob/v5.1.0/activerecord/lib/active_record/statement_cache.rb
https://github.com/rails/rails/blob/v5.1.0/activerecord/lib/active_record/connection_adapters/statement_pool.rb

The `:doc:` was added in bc478158 but originally `UriEncoder` is a
`:nodoc:` class.

[ci skip]

Merge pull request #28936 from dixpac/dixpac/update-docs-ActionDispatch__Journey__Router__Utils.unescape_uri

Add docs for  Router::Utils.unescape_uri method. [ci skip]

Fix note about `secrets.yml` [ci skip]

The `secrets` method is in `Application` class, not `Configuration` class.

* Remove trailing spaces.
* Add backticks around method and command.
* Fix indentation.

Remove entries included in 5.0 [ci skip]

* `Permit same-origin connections by default` was backported in db709789, 549d7322.
* `Deprecated `:controller` and `:action` path parameters` was included in 6520ea5f.
* `Exception handling: use `rescue_from` was included in 6b543166.

Stop creating duplicate Struct instances

Just use one `Event` class. Reduces duplication, makes the tests easier
to read. It might seem like each tests needs a different kind of Struct,
since we make a new one for each test case.

Add missing require for `remove_possible_method`

https://github.com/rails/rails/commit/505537082849d912e8e29819655b80a573e93c0c
added a call to `remove_possible_method`, but didn't require the file
that defines it.

@prathamesh-sonpatki added the last missing commits in #28911.

[ci skip]

[ci skip]

Will backport to 5-1-stable, since the original PR landed there.

[ci skip]

Fix indentation [ci skip]

Added some more release notes for Active Record [ci skip]

Ensure input to distance_of_time_in_words is not nil