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.
probably cleaner to do something like:

```
if sr_id:
    q = q._filter(kind.c.sr_id == sr_id)
````