Fix typos in Rails 5.0 release notes [ci skip]

Fix Regression in Numericality Validations

Fix letter case and grammar in Routing guide

A regression (#22744) introduced in 7500daec caused certain numericality
validations to raise an error when run against an attribute with a
string value. Previously, these validations would successfully run
against string values because the value was cast to a numeric class.

This commit resolves the regression by converting string values to
floats before performing numericality comparison validations.

[fixes #22744]

Covers Regressions:

* <=
* <
* ==
* >
* >=
* other than

`join_to_delete` is same as `join_to_update`

Add #== back to ActionDispatch::MiddlewareStack::Middleware

Add support for passing flags to MySQL2 adapter by array

seeds.rb.tt: fix English.

fixes typo in 5.0 release notes

remove cors initializer from rails app

[ci skip]

[ci skip]

[ci skip]

Mounting it as prescribed here:

https://github.com/rails/rails/blob/0d1d50c2db40ea1a9d3bebfb0c35da43bbd4d27f/railties/lib/rails/generators/rails/app/templates/config/routes.rb#L5

[ci skip]

`rack-cors` gem is defined in Gemfile by default only if the api,
not defined by default in rails app.

xref #22701.
xref #20612.

[WIP] Rails 5 release notes [ci skip]

use `bin/test` in plugins guide [ci skip]

do not use `div_for` in example [ci skip]

remove content_tag_for example

Fix .keep file issue with ActionCable

Fixes #22708
Make sure the channels file is create in assets/javascript/ so require
tree works in asset pipeline.

`div_for` removed in 01e94ef3

corrects a typo [ci-skip]

Clarify config settings for AS::halt_callback_chains_on_return_false

Fix Rails tutorial link

[ci skip]

The old link is 404 not found.
The link matches what the rest of the documentation already points to:

* https://github.com/rails/rails/blame/2de2263118d6763c11a02db05079a5b7a97a0a34/guides/source/getting_started.md#L2052
* https://github.com/rails/rails/blame/2de2263118d6763c11a02db05079a5b7a97a0a34/README.md#L74

This was causing bug #22738 to occur. Also added extra tests to make
sure everything is A-OK.