Use mw-mmv CSS prefix consistently for all things

With apologies to anyone who gets a hundred merge conflicts
because of this :)

We had several different prefix styles (mlb-, mw-mlb-, mw-mmv-,
mw-mmv-mmv-, a few unprefixed), which was getting annoying,
and will be confusing to wiki editors who are trying to figure out
where a given style comes from. Such changes are better done before
going live because it breaks all local CSS tweaks on the wiki,
so I am renaming things now (also removing some stuff which wasnt
used anywhere).

Change-Id: I00447a25f0028e234169c6db941bedc99622eb8d
Merge "Use mw-mmv CSS prefix consistently for all things"