Rake restart task no longer loads entire Rails environment when run

The restart task does not need access to models or other classes and
helpers from the application environment.

[Robin Dupret + Rafael Mendonça França]

Improve guides [ci skip]

[ci skip] Add code example for MailHelper#block_format documentation

Nit pick the byebug settings guide for default values [ci skip]

Web Console documentation for the upcoming 2.1 release [ci skip]

In #19097 we had a discussion where we decided to change `byebug`
settings sections default part to actually say `defaults`.

With the upcoming 2.1 [web-console release], I have tweaked the existing
documentation a bit. I tried to focus on the spawning console use case,
because I think a lot of people overlook that.

Tried to explain it as best as I can, however, my English and prose
aren't my best skills :) If you guys can chime in and help me improve
the wording, I would be extremely grateful.

There has been some configuration default changes. For example, the
whitelisted IPs always include IPv4 and IPv6 localhosts now and this
wasn't the case in 2.0. I think a lot of people got bitten by it, that's
why I changed it. I'm a bit confused on how to document this. Should I
just document the latest version, I don't think I can expect all the
people reading the guide to be on it.

[web-console release]: https://github.com/rails/web-console/pull/110

On guides, correcting incorrect controller name used on routing.md.

On "Controller Namespaces and Routing" section of the "Rails Routing from the Outside In" , the examples are using "AritclesController" as an example of a controller, to show different organizers. But on the line 232, it incorrectly uses "PostsController" instead of "AritclesController".

Use include? instead of in? for Enumerable#without.

[docs fix] Composite index is actually [:imageable_type, :imageable_id]

[egilburg]

As far as I can tell nobody is setting this variable.

Include object inclusion for enumerable test.

Add Enumerable#without

`@reflects_state[depth+1]` will always be nil because we haven't made a
method call that would make it true yet.

Removing unused mailer templates

```ruby
article = Article.new.tap(&:save!)
view.url_for article

result = ObjectSpace::AllocationTracer.trace do
  3000.times { view.url_for article }
end
p ObjectSpace::AllocationTracer.allocated_count_table[:T_STRING] / 3000
```

I found that nil values were being incorrectly sent to
`parse_nested_query` in Rack. Originally it was thought that Rails was
doing something incorrect but it was actually rack-test. This was fixed
in brynary/rack-test@4a4b2c1 and is no longer an issue. This commit
bumps rack-test in Rails so changes to Rack don't cause failures.

See rack/rack#813 for more information.

_generate_paths_by_default wasn't used in AD::Routing::UrlFor, so we
should be able to move it where it is used in AV::Routing

[docs fix] Indexing a polymorphic assoc adds index on type and id

[ci skip] Fix fenced code block lang

[ci skip] Add `.`

Actually, our app has 400,000 LOC 💥

Introduced in f6e293ec ActionPack tests began sefaulting. I found that it
was the kwargs and the test causing the seg fault was missing the new
default_headers argument.