[DEPRECATING CHANGE] Rename the 'MediaWiki' theme to 'WikimediaUI'

Authored by Jdforrester-WMF on May 16 2017, 4:01 PM.

Description

[DEPRECATING CHANGE] Rename the 'MediaWiki' theme to 'WikimediaUI'

Bug: T164613
Change-Id: I8ee1fab4fc6aefca1ee48db0cd64a6caf18f1757