Move desktop->mobile diff redirect logic from to Special:MobileDiff

* somewhat generecizes desktop -> mobile redirect method in
MobileContext for potential similar cases in the future
* simplifies unit tests
* Move MFMockRevision out of its own file into SpecialMobileDiffTest.php

Change-Id: Ide0b9d13e2504c186c91640ce29466a0c3ef2d3e