Cache the value of `changed_attributes` when calling `changes_applied`

* Also duplicated find_by tests from relations_test.rb to finder_test.rb now
	that we have a completely different implementation on the class (in core.rb
	with AST caching stuff).

* Also removed a (failing) test that used mocks. Now that we have tests for the
  behavior, there's no point having another test that tests the implementation
	(that it delegates). Further, what the test was implying is nolonger true with
	the current implementation, because Class.find_by is a real method now.

Added web links

Added web links to resque, delayed_job and sidekiq

[ci skip] make assert messages consistent

Use ActiveSupport::Concern

Fix a few typos and made some tiny improvements. Refs #16576.

[Zachary Scott & Robin Dupret]

[ci skip] use proper apostrophe

Provide a builder for form labels to customize wrapping around I18n content

This is a reacon to https://github.com/rails/rails/commit/d6c1205584b1ba597db4071b168681678b1e9875#commitcomment-7502487
This backwards incompatibility was introduced with d6c12055 to fix #7516.
However both `connection.default_sequence_name` and `model.sequence_name` are public API.
The PostgreSQL adapter should honor the interface and return strings.

/cc @matthewd @chancancode

[ActiveJob] Add activesupport as dependency [ci skip]

Add I18n support for `:placeholder` HTML option is passed to form fields

Fix `ActiveSupport::Duration#instance_of?` to behave like `#is_a?`

Address comments on Gzip implementation

treetop is no longer a mail (rails) dependency [ci skip]

- don't mutate PATH_INFO in env, test
- test fallback content type matches Rack::File
- change assertion style
- make HTTP_ACCEPT_ENCODING comparison case insensitive
- return gzip path from method instead of true/false so we don't have to assume later
- don't allocate un-needed hash.

Original comments:

https://github.com/rails/rails/commit/
cfaaacd9#commitcomment-7468728

cc @jeremy

see https://github.com/mikel/mail/commit/2da7c7985c221272f6451b27ab8b41e84e0a6804

* Remove the libxml2 and libxslt part ; Nokogiri ships now with these
  libraries bundled.
* Add the missing installation instructions for either Arch Linux,
  FreeBSD or Mac OS X.
* Use the new `pkg install` syntax over `pkg_add -r` for FreeBSD 10+
  users.
* Point to the up-to-date version of PostgreSQL as for the BSD port.
* Make the platform installation instructions order consistent
  throughout the guide.
* Remove useless duplicated links to Homebrew's home.

[ci skip]

show stats for app/jobs

[ci skip] Added documentation for has_many extension parameter

Grammer and spelling pass on web console release note. [CI SKIP]

Since Rubinius is relying on #instance_of? for its definition of #eql?
(http://git.io/MtmbbA) but ActiveSupport::Duration should behave like
is_a? it returns true with `Fixnum`.

Thus, for the moment, the last assertion is failing so we have to skip
this test.

For the sake of backward-compatibility, we need to make #instance_of?
return true for Fixnum. On the other hand, the method should still
give true for ActiveSupport::Duration itself which was not the case
before.

Describe web-console in the release notes

Raise ActiveJob::SerializationError when cannot serialize job arguments

I put a more up-to-date description of Web Console in the release notes.
I'm not really happy with the language, so if you guys can help me with
that I'll be grateful :)

Fix guide anchor generation logic.

* Require either FIRST or LAST qualifier for "NULLS ..."
* Require whitespace before "NULLS ..."