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 ..."

Fix postgresql adapter #columns_for_distinct trimming whitespace it shouldn't

This could use in any translation because it's not limited to `[^a-z0-9]`.

Verify after this change, all guides still have same anchors.

[Robin Dupret & Juanito Fatas]

Fixes #16623 introduced by https://github.com/rails/rails/commit/3d5a2019bcccc6fb01bee4811ca669f4383edb51

Fix example code in Active Job guide [ci skip]