Allow sorting userpage overview, submissions, and comments by subreddit

This should allow users with gold to sort their userpage by subreddit in order to view comments, submissions, or both, from just a specific subreddit. This is similar to sorting saved links by subreddit.
That is supposed to get the list of subreddits that the user has submitted to, which if im modifying overview and comments as well i would have to modify that for comments as well, conditionally.