Missing backquotes in the guides

link tags shouldn't depend on the request

Use __callee__ to pass alias instead of original method name

ActionCable: use find method when unsubscribing

Before

```
> Article.left_joins
ArgumentError: The method .left_outer_joins() must contain arguments.
```

After

```
> Article.left_joins
ArgumentError: The method .left_joins() must contain arguments.
```

Implement H2 Early Hints for Rails

When puma/puma#1403 is merged Puma will support the Early Hints status
code for sending assets before a request has finished.

While the Early Hints spec is still in draft, this PR prepares Rails to
allowing this status code.

If the proxy server supports Early Hints, it will send H2 pushes to the
client.

This PR adds a method for setting Early Hints Link headers via Rails,
and also automatically sends Early Hints if supported from the
`stylesheet_link_tag` and the `javascript_include_tag`.

Once puma supports Early Hints the `--early-hints` argument can be
passed to the server to enable this or set in the puma config with
`early_hints(true)`. Note that for Early Hints to work
in the browser the requirements are 1) a proxy that can handle H2,
and 2) HTTPS.

To start the server with Early Hints enabled pass `--early-hints` to
`rails s`.

This has been verified to work with h2o, Puma, and Rails with Chrome.

The commit adds a new option to the rails server to enable early hints
for Puma.

Early Hints spec:
https://tools.ietf.org/html/draft-ietf-httpbis-early-hints-04

[Eileen M. Uchitelle, Aaron Patterson]

Encourage html-safe API in layouts/rendering guide

Fix formatting of `Time.use_zone` [ci skip]

* Add missing credit
* Add backticks
* Fix indentation
* Remove trailing spaces

And some minor tweaks.

Fix third-party system libraries list in ActiveStorage::Preview [ci skip]

Add test case for `font_url`

Since test of `font_url` was not in Action View's test suite, so it added.

rails-ujs: Update README

Make various wording tweaks to cater to users who are viewing the README
on NPM. Notably, don't highlight Yarn specifically in the installation
instructions -- even though this is the preferred tool of choice
especially in the Ruby community, some people still use NPM (and,
really, ES2015+ syntax has nothing to do with NPM or Yarn).

Add link to Rack website [ci skip]

Resemble links to Bundler and Yarn,
based on feedback by @kamipo

[ci skip]

Remove unused `new_credentials_configuration`

`new_credentials_configuration` is no longer used since 081a6ac6f7fd929798481f9ee333fb92b441356c.

Fix broken link to recaptcha.net [ci skip]

The link to recaptcha.net returns a 404. As far as I can tell, the new
link ought to be to https://developers.google.com/recaptcha/ .

Currently `:api:` tag has leaked on the doc directly since RDoc doesn't
support `:api:` tag directive.

http://api.rubyonrails.org/v5.1/classes/AbstractController/Rendering.html

So `:api: private` doesn't work as expected. We are using `:nodoc:` for
the purpose.

Related #13989.

Remove unused methods from `RenderPartialWithRecordIdentificationController`

Follow up of #30728.

These methods no longer used since a3da293b.

Don't use Active Support where it is not needed.

Fix "warning: method redefined; discarding old test_scaffold_plural_names"

This code has been changed with https://github.com/rails/rails/pull/30735/files#diff-8e5f6b33c191ad6dec07f3288345a13fL47.

However, `active_support/time` is not load automatically, so if use
Action Pack alone, `days` method can not use and an error occurs.
In this case, I think that there is no problem by specifying a value
with Integer.