Resolves #24924.

References https://github.com/rails/homepage/issues/46.

This reverts commit 8c3cca5e, reversing
changes made to 9dcf67c4.

Reason: https://github.com/rails/rails/pull/23562#issuecomment-181442569

rails-html-sanitizer 1.0.2 is vulnerable: https://groups.google.com/d/msg/rubyonrails-security/uh--W4TDwmI/m_CVZtdbFQAJ

It is already on Active Support

The concurrent-ruby gem is a toolset containing many concurrency
utilities. Many of these utilities include runtime-specific
optimizations when possible. Rather than clutter the Rails codebase with
concurrency utilities separate from the core task, such tools can be
superseded by similar tools in the more specialized gem. This commit
replaces `ActiveSupport::Concurrency::Latch` with
`Concurrent::CountDownLatch`, which is functionally equivalent.

and fix the grammar in the ruby_version_check.rb user message.

Prevents double escaping errors, such as "&" becoming "&".

This is a follow-up to #19257

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.

Rack is very carefully released, we should be able to upgrade minor
versions without much effort. We are a bunch of Rails core who are also
Rack core members so there won't be any issue with that. And in case
there's something wrong, we should fix on both sides.
Even though, doesn't seem like we will have a 1.7 version, this will be
useful as an example for when we go with Rack 2.0. We should ~> 2.0.

Stems from [this comment](https://github.com/rails/rails/pull/18203#issuecomment-68138096) by @robin850
and by the blog post http://weblog.rubyonrails.org/2014/12/19/Rails-4-2-final

`rack.version` was bumped in https://github.com/rack/rack/commit/28e77c710ac2ddb35c436d78d72e28f28f964446

[This article](http://weblog.rubyonrails.org/2014/8/20/Rails-4-2-beta1/#maintenance-consequences-and-rails-5-0) states that:

> Rails 5.0 is in most likelihood going to target Ruby 2.2.

Before the exact minimum version is fully decided, @arthurnn [suggests](https://github.com/rails/rails/pull/17830#issuecomment-64940383)
that **at least** version 2.1.0 **must** be required by the `gemspec` files.

- Default to Rails::DeprecatedSanitizer in ActionView::Helpers::SanitizeHelper.
- Add upgrade notes.
- Add sanitizer to new applications Gemfiles.
- Remove 'rails-dom-testing' as a dependency.

Implemented ActionView: FullSanitizer, LinkSanitizer and WhiteListSanitizer in sanitizers.rb.
Deprecated protocol_separator and bad_tags.
Added new tests in sanitizers_test.rb and reimplemented assert_dom_equal with Loofah.

As Rack has some non backwards compatible changes added required
modifications to keep behaviour in rails close to same as before.

Also modified generators to include rack/rack for not yet released
version of rack

This gem is used by Active Support but it should not be a dependency of
Action Pack.

This commit creates structure for Action View gem and is first of a
series of commits extracting Action View from Action Pack.

This reverts commit c24528fb.

All ActionPack and Railties tests are passing. Closes #8891.

[Carlos Antonio da Silva + Santiago Pastorino]