Update comment to reflect that unscoped works with named scope
even when named scope is using without block form

It's sometimes hard to quickly find where deprecated call was performed, especially in case of migrating between Rails versions. So this is an attempt to improve the call stack part of the warning message by providing caller explicitly.

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

Don't use this:

    scope :red, where(color: 'red')
    default_scope where(color: 'red')

Use this:

    scope :red, -> { where(color: 'red') }
    default_scope { where(color: 'red') }

The former has numerous issues. It is a common newbie gotcha to do
the following:

    scope :recent, where(published_at: Time.now - 2.weeks)

Or a more subtle variant:

    scope :recent, -> { where(published_at: Time.now - 2.weeks) }
    scope :recent_red, recent.where(color: 'red')

Eager scopes are also very complex to implement within Active
Record, and there are still bugs. For example, the following does
not do what you expect:

    scope :remove_conditions, except(:where)
    where(...).remove_conditions # => still has conditions

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.