Fix sort on category pages

Sorting by most upvoted, most downvoted and most commented did not work when viewing a specific category.

This change sets up the `index_url_upvoted`, `index_url_downvoted` and `index_url_commented` smarty variables on category pages in the same way as the date sorting variables.
Merge pull request #149 from mcs07/categorysort

Fix sort on category pages