Simplify MediaWiki UI codebase

* Stop thinking in terms of skins. Apply same styles to all uses of
  MediaWiki. MediaWiki UI should be able to be slotted in anywhere.
* Kill unused mixins
* Move variables into single separate file
** Kill unused variables
* Do not load variables in mixin files (skins should tweak if necessary)
* Create a single variables file

Change-Id: Ib6c2485aca9a6a7c88f1a5326d9990647a1a26fb