Change User association on Comment model

[luaybs]

This PR will allow multiple user types to comment (Admins, Users, etc.) on a commentable object.

I changed the user association on the comment model to a polymorphic commenter.
I updated the migrations and comment model templates for both the fresh install and upgrading from the acts_as_commentable gem.

[luaybs]

I fixed the merge conflicts with your master branch. :)