Add M.tapEvent (temporarily)

Instead of writing mw.config.get( 'wgMFMode' ) === 'alpha' ? 'tap' : ...
If we push micro.tap to stable, this can be removed.

Change-Id: Ib3b4dc4ef549195b3f0c2fa790b6d42bd129499a
Merge "Add M.tapEvent (temporarily)"