Ensure app/mailers is removed when skipping action-mailer

Closes #26145

Add documentation about `ActiveSupport.on_load`

[ci skip]

This commit adds some docs that explain how `LazyLoadHooks.on_load` method
works.

Use `ActiveRecord::TestCase` rather than `ActiveSupport::TestCase` in AR test cases

Make cache max-age durations self-explanatory

Pass over changelogs

They are not part of the public API

Update example in Http::Headers object to use `.from_hash`

When initializing an `ActionDispatch::Http::Headers` object it takes a request object (Rails 5) whereas before it took a hash (Rails 4.x) but the documented example still shows a hash given to the constructor (due to commit 34fa6658) so this is just a documentation change to use the new `from_hash` method introduced in that earlier commit.

Ensure values are strings before calling gsub

[ci skip] Fix the wrong ActionCable documentation in the guide.

[ci skip] Add documentation to Parameter Encoding

Fix unsafe query generation risk.

Redo of CVE-2012-2660, CVE-2012-2694 and CVE-2013-0155

CVE-2016-6317

Creating a new Topic class instead of class_eval for the existing one

Fix warning: ambiguous first argument

Many helpers mark content as HTML-safe without escaping double quotes -- including `sanitize`. Regardless of whether or not the attribute values are HTML-escaped, we want to be sure they don't include double quotes, as that can cause XSS issues. For example: `content_tag(:div, "foo", title: sanitize('" onmouseover="alert(1);//'))`

CVE-2016-6316

since it affects another test `ReflectionTest#test_read_attribute_names`

Address #26099

add missing require rake

In ff8035df, require rake is deferred.
Therefore, it is necessary to require rake even `Engine::CommandsTasks.

Fix actionview test failure

Caused by #26092.

Minor doc fix related to ActiveModel::SecurePassword [ci skip]

Thinking .. relative to files is not natural, we are used
to think "parent of a directory", and we have __dir__
nowadays.

Add documentation for `http_cache_forever`. [ci skip]

Allow specifying encoding of parameters by action

Fix a NoMethodError schema_statements.rb

In c546a2b0 parameter handling in AC test cases was changed to
round tripping through encoders/decoders so that they matched
reality and in 0adb8f8f the old methods were removed but the
`html_format?` method was overlooked.

If you call `remove_index` with wrong options, say a type, like I did,
you get:

```
== 20160810072541 RemoveUniqueIndexOnGoals: migrating =========================
-- remove_index(:goal, {:coulmn=>:kid_id, :unique=>true})
rails aborted!
StandardError: An error has occurred, this and all later migrations canceled:

undefined method `ArgumentError' for #<ActiveRecord::ConnectionAdapters::PostgreSQLAdapter:0x007fb7dec91b28>
```

What happened is that I mistyped column (coulmn) and got a
`NoMethodError`, because of a missing comma during the raise. This made
Ruby think we're calling the method `ArgumentError`.