Add a note explaining when the has_many associations callbacks will be called or not.

Add version awareness to rails db:system:change

Build assets automatically if necessary

In CI, these are unnecessary because done by preprocessing.
To test locally, you need to do it manually. If assets not exist, it will
fail when a run specific test, so force to build those only when necessary.

Update `multiline?` sample code [ci skip]

Corrected execution result of Enumerable#sum [ci skip]

Delegate `only` query method to relation as with `except`

Learn more about inflections[ci skip]

Update racc to latest version

racc 1.4.15 includes fixes for compiling with Ruby 2.7 since
[ruby/ruby@3d1c86a](https://github.com/ruby/ruby/commit/3d1c86a).

Before:

```
ruby -v
ruby 2.7.0dev (2019-03-07 trunk 67189) [x86_64-linux]

bundle install
...
Fetching racc 1.4.14
Installing racc 1.4.14 with native extensions
Gem::Ext::BuildError: ERROR: Failed to build gem native extension.

current directory:
/home/u/.rbenv/versions/2.7.0-dev/lib/ruby/gems/2.7.0/gems/racc-1.4.14/ext/racc
/home/u/.rbenv/versions/2.7.0-dev/bin/ruby -I
/home/u/.rbenv/versions/2.7.0-dev/lib/ruby/2.7.0 -r
./siteconf20190307-14242-16jzp6c.rb extconf.rb
checking for rb_ary_subseq()... yes
creating Makefile

current directory:
/home/u/.rbenv/versions/2.7.0-dev/lib/ruby/gems/2.7.0/gems/racc-1.4.14/ext/racc
make "DESTDIR=" clean

current directory:
/home/u/.rbenv/versions/2.7.0-dev/lib/ruby/gems/2.7.0/gems/racc-1.4.14/ext/racc
make "DESTDIR="
make: *** No rule to make target
'/home/u/.rbenv/versions/2.7.0-dev/include/ruby-2.7.0/defines.h', needed
by
'cparse.o'.  Stop.

make failed, exit code 2

Gem files will remain installed in
/home/u/.rbenv/versions/2.7.0-dev/lib/ruby/gems/2.7.0/gems/racc-1.4.14
for
  inspection.
  Results logged to
  /home/u/.rbenv/versions/2.7.0-dev/lib/ruby/gems/2.7.0/extensions/x86_64-linux/2.7.0-static/racc-1.4.14/gem_make.out

  An error occurred while installing racc (1.4.14), and Bundler cannot
  continue.
  Make sure that `gem install racc -v '1.4.14' --source
  'https://rubygems.org/'`
  succeeds before bundling.

  In Gemfile:
    racc
```

I've found the skewness of delegation methods between `except` and
`only` in a88b6f25.

The `only` method is closely similar with `except` as `SpawnMethods`.

https://github.com/rails/rails/blob/e056b9bfb07c4eb3bcc6672d885aadd72bec574f/activerecord/lib/active_record/relation/spawn_methods.rb#L53-L67

It is preferable both behaves the same way.

Replace “can not” with “cannot”.

This makes to ease testing `QUERYING_METHODS`.

Explicitly require rails-html-sanitizer gem in ActionText helpers

[ci skip] Fix typos

That is happened with `distinct`, and that is not only happened on MyS
L, but also on PostgreSQL, SQL Server, and Oracle.

[ci skip]

[ci skip]Fix typo:  constaint -> constraint

If the [`action_text.helper` initializer][0] runs after
`ActionController::Base` has been loaded, but before the
`rails-html-sanitizer` gem has been `require`d, then the reference to
the constant `Rails::Html` in the body of the
`ActionText::ContentHelper` module raises an `uninitialized constant`
exception.

[0]: https://github.com/rails/rails/blob/21703382393c87212c27c988420ee5c133c1aa9f/actiontext/lib/action_text/engine.rb#L31-L35

Fix including/excluding flattening

Update links and code examples in the guides to use HTTPS

Load YAML for rake tasks without parsing ERB

Although the old name had a certain persistence, this ain't the kind of
file we're in now.

Foreign keys could be created to the same table.
So `remove_foreign_key :from_table, :to_table` is sometimes ambiguous.
This allows `remove_foreign_key` to remove the select one on the same
table with giving both `to_table` and `options`.

This change adds a new method that loads the YAML for the database
config without parsing the ERB. This may seem odd but bear with me:

When we added the ability to have rake tasks for multiple databases we
started looping through the configurations to collect the namespaces so
we could do `rake db:create:my_second_db`. See #32274.

This caused a problem where if you had `Rails.config.max_threads` set in
your database.yml it will blow up because the environment that defines
`max_threads` isn't loaded during `rake -T`. See #35468.

We tried to fix this by adding the ability to just load the YAML and
ignore ERB all together but that caused a bug in GitHub's YAML loading
where if you used multi-line ERB the YAML was invalid. That led us to
reverting some changes in #33748.

After trying to resolve this a bunch of ways `@tenderlove` came up with
replacing the ERB values so that we don't need to load the environment
but we also can load the YAML.

This change adds a DummyCompiler for ERB that will replace all the
values so we can load the database yaml and create the rake tasks.
Nothing else uses this method so it's "safe".

DO NOT use this method in your application.

Fixes #35468

[ci skip] Fix typo `beacuse` -> `because`