remove old DB migration for AA comments

Now when you create a new project with AA, only one DB migration is
used to create comments
bug fixes for AA Comments UI

+ fixes #367 regression by calling `to_s` on the namespace
+ fixes #2294 by applying queries to the passed object
+ disables eager load of polymorphic relationships for Rails 3.0 since they don't work
cleans up `active_admin_comments_for`