ActiveUsersPager: Fix ordering and return 0-action users
Ia9d2ff00 introduced two incompatibilities with what the base classes
were trying to do:
- The $descending flag passed to buildQueryInfo() does not actually mean DESC if $this->mIsBackwards is set. The old code just blindly assumed that was the case.
- For paging to work right, we must return all $limit rows, we can't filter out users with 0 actions. Thus we need to LEFT JOIN recentchanges (and move some conditions around to cope)..