Callback caller class uses `after_initialize`,
but Callback callee defines `after_find`.

Current sample code causes following error.

  NoMethodError: undefined method `after_initialize' for #<EncryptionWrapper:0x007fe4931fa5c0>

Get rid of ActiveModel::Configuration, make better use of
ActiveSupport::Concern + class_attribute, etc.

This reverts commit fa6d921e.

Reason: Not a fan of such massive changes. We usually close such changes
if made to Rails master as a pull request. Following the same principle
here and reverting.

[ci skip]

For future reference, this is the regex I used: ^\s*#\s*\n(?!\s*#). Replace
with the first match, and voilà! Note that the regex matches a little bit too
much, so you probably want to `git add -i .` and go through every single diff
to check if it actually should be changed.

This reverts commit 55df1df9.

The problem: We need to be able to specify configuration in a way that
can be inherited to models that include ActiveRecord::Model. So it is
no longer sufficient to put 'top level' config on ActiveRecord::Base,
but we do want configuration specified on ActiveRecord::Base and
descendants to continue to work.

So we need something like class_attribute that can be defined on a
module but that is inherited when ActiveRecord::Model is included.

The solution: added ActiveModel::Configuration module which provides a
config_attribute macro. It's a bit specific hence I am not putting this
in Active Support or making it a 'public API' at present.

Signed-off-by: NSantiago Pastorino <santiago@wyeworks.com>

Signed-off-by: NJosé Valim <jose.valim@gmail.com>

Revert "documentation regarding after_validation"

This reverts commit c24aa2f2.

[#4653 state:resolved]
Signed-off-by: NJosé Valim <jose.valim@gmail.com>

say something about after_(commit|rollback) in callbacks.rb, the fact that their implementation is elsewhere is not important for rdoc purposes