This commit adds `AbstractController.deprecator` and
`ActionController.deprecator`, and replaces all usages of
`ActiveSupport::Deprecation.warn` in `actionpack/lib/action_controller`
with the latter.

Additionally, this commit adds `ActionController.deprecator` to
`Rails.application.deprecators`.  Because `AbstractController` does not
have its own railtie to do the same, `AbstractController` and
`ActionController` use the same deprecator instance.  Thus, both can be
configured via `Rails.application.deprecators[:action_controller]` or
via config settings such as `config.active_support.report_deprecations`.