Add new 'mediawiki.skin_diff' event logging stream

Per T289622, we should rename mediawiki.pref_diff to
mediawiki.skin_diff. In an effort to prevent data loss, a new stream
of this name was created and the mediawiki.pref_diff stream will be
removed in a follow up patch.

Bug: T289622
Change-Id: I48e212a81bcc0d0270f2c850541d26503ca5881c