Prefer WikimediaEvents to CoreEvents, now that the extension has been renamed.

'CoreEvents' has been renamed to 'WikimediaEvents'; this change makes the
transition easier to manage by conditionally loading WikimediaEvents if it is
present but falling back to CoreEvents if not.

Change-Id: If01b2a6ca6511a518fe91355a27ec1af0dd845ea
Merge "Prefer WikimediaEvents to CoreEvents, now that the extension has been renamed."