Revert "Remove Special:ActiveUsers"

This reverts commit 27104686abe2aaf464d8d654a71e946326a0926d.

Revert "Remove link to Special:ActiveUsers from Special:Statistics"

This reverts commit 62949af09ea3ceb96d897ec8aab74afdadb02d4e.

I used a sledgehammer where a scalpel would've done--and I should
be whacked with the cluebat. This should be done per-wiki, plus
improvements.

Change-Id: Ib43b42057b8d9f714a8a71ba9cbe375385254b7c