add category slug field to comments to deal with the lack of polymorphic eager
loading in the current version of laravel 4. theoretically, this could later be
removed and replaced by an eager load of the 'owner' relationship for the
comments. This would add a single query but make the database more normalized. I
personally don't hvae a problem with the category_slug field. However, a
developer coming into the project later could be confused as to why it's there
and then trace down the rabbit hole for an hour until they figure it out.

this commit also stabilizes the number of queries on the forum category view
page