Explain that default_scope also influences update_all

This was not explicitly stated before and I needed to try it out to be
certain. A little explicit statement in the API docs might help here.
Fix includes on association with a scope containing joins along with conditions
on the joined assoiciation
Creates an ApplicationMailer and layout by default, including html and body tags to reduce spam score
Move test that depends on ActiveRecord

This commit moves a test from `test/template` to `test/activerecord` since the
test depends on ActiveRecord.

This matches the documentation from [RUNNING_UNIT_TESTS](https://github.com/rails/rails/blob/f28d1ddd507174ac233b773cc4f35c3c05ad32e7/actionview/RUNNING_UNIT_TESTS.rdoc):

> Test cases in the test/activerecord/ directory depend on having activerecord and sqlite3 installed.
> If Active Record is not in actionview/../activerecord directory, or the sqlite3 rubygem is not installed, these tests are skipped.
> Other tests are runnable from a fresh copy of actionview without any configuration.

---

More details about this commit.

All the tests starting with `require 'active_record_unit'` are already
inside `test/activerecord`, except for the one test this commit moves.

If you don't have `active_record` on your machine, the following command
currently fails on master:

```bash
```
Fix cases where the wrong name is passed to `Formatter#generate`

These are currently working "by accident" because `match_route` does not check
that the name is valid.
Deprecate passing an invalid name to `Formatter#generate`

The internal tests that (incorrectly) relied on this were already fixed in
938d130. However, we cannot simply fix this bug because the guides prior to
b7b9e92 recommended a workaround that relies on this buggy behavior.

Reference #17453
Deprecate `use_route` in controller tests

Reference #17453
Don't show the warning if we're already raising the error anyway

If the route set is empty, or if none of the routes matches with a score > 0,
there is no point showing the deprecation message because we are already be
raising the `ActionController::UrlGenerationError` mentioned in the warning.

In this case it is the expected behavior and the user wouldn't have to take any
actions.
Removing unnecessary File.join calls
Replace ActionDispatch::Head with Rack::Head.
Reword documentation for update_all

It now contains a carefully formulated reference to the "current relation" which might help clarify that the receiving will generate its own scope, escaping the need for explicitly referencing `default_scope` which is, after all, just another way of specifying a scope and nothing special.
Change 'of plugins' syntax

Currently, the docs uses a syntax that is unclear and not general
American English. I've switched it to be clearer wording. Not a big
fix, but may be helpful.
Merge pull request #17742 from RichardLitt/doc-fix-syntax

Change 'of plugins' syntax
Fix wrong path in comments about stylesheets

I put the wrong path in my last PR by accident. Fixed here. Related to #17742
Merge pull request #17744 from RichardLitt/doc-fix-syntax

Fix wrong path in comments about stylesheets
Merge pull request #17707 from claudiob/move-debug-helper-test

Move test that depends on ActiveRecord
Merge pull request #17678 from siddharth28/includes_with_scope_with_joins

Fix includes on association with a scope
Merge pull request #17737 from yui-knk/fix/head

Replace ActionDispatch::Head with Rack::Head.
Merge pull request #17725 from chancancode/deprecate_use_route

Deprecate use route
Ensure the type map's cache is thread safe

Thanks to @thedarkone for pointing out that an instance of this object
is used in a shared context.