Preferences: Disable the 'skin' preference if there are no skins

Having a radio field with no valid values makes MediaWiki sad and
causes exceptions.

Also make double-sure that the global default value is not invalid.
That causes exceptions too.

Change-Id: I90cc9e6f40303aa5771c265948a0be4a4ce2e42c
Merge "Preferences: Disable the 'skin' preference if there are no skins"