ActiveUsersPager: Fix ordering and return 0-action users

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)..

Bug: T217525
Change-Id: Iba105a31ff61fbb14931bf8903538bfe7b0ba48b