Fix stored configs with ActiveRecord 4.x

Add a patch restoring the stored configs functionality with Jessie's
ActiveRecord 4.x:

 - Use AR::Base.clear_active_connections! instead of
   AR::Base.verify_active_connections!

 - Always call AR::Base.connection as a class method, never as an
   instance method.

 - Require 'activerecord/deprecated_finders' to make all #find(:all)
   and #find_by_x methods work again.

 - Silence AR's deprecation warnings. We know we are using deprecated
   finders so these warnings are just (a lot of) noise.

 - Drop all ":integer => true" options passed to add_index in the
   schema. This was probably never a valid option anyway and was ignored
   until Rails 4.x, but currently causes the DB bootstrapping to fail.

Closes: #774643