Now we keep only the common code and move the specific code to the gems

[ci skip]

See
https://github.com/rails/turbolinks/commit/153f1b0f04c718442cfd73365a2778dfe1a1c5c7

Since the rubysl-yaml gem doesn't ship with Psych by default because of
its dependency on libyaml, on Rubinius, the default engine is Syck.

However, if we want to be able to run the application safely on
different rubies, we need to make people using Rubinius rely on Psych.

See http://git.io/uuLVag for further information.

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.

Completes https://github.com/rails/rails/pull/15493

Revert "For our build, stick with mail 2.5.x for now"

This reverts commit b8f586a0.

2.6 currently has many warnings, which are failing the Travis build.

Delayed jobs are supported by all systems except QueueClassic.  For it I decided to raise NotImplementedError.

The inline implementation is a bit rough.