Enable RevisionDelete

Summary:
This feature allows sysop to hide revision text from and edit
summary for users, so they can clean copyright issues.

The Utopia wiki wants also to use this feature to hide some
remaining revisions from a 2011 spam attack.

Reference: https://www.mediawiki.org/wiki/Manual:RevisionDelete

Test Plan: Tested on Utopia wiki

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D1514