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

Special:ActiveUsers still seems to be in REL1_21, and
there is a pending change set (Ib43b4205) to add back
the special page on the master branch.

This reverts commit 4b2c7373f2ab6f701cbd7f371ad4b95829e34e70

Change-Id: I5669477091ada36ea28c33de1232d2b7e9d0b413
Merge "Revert "Remove link to Special:ActiveUsers from Special:Statistics"" into REL1_21