Add sortby last activity as a user preference

To sort by last activity, pass in topiclist_sortby=updated.
To save it in user's preference for non-anon users, also pass
in topiclist_savesortby=true

To clear sort by last activity, pass in topiclist_sortby=

Change-Id: I78dd2d5509876d91137a429b8f421e50f4476c47
Merge "Add sortby last activity as a user preference" into frontend-rewrite