Rename basic skinning modules: skins.common.* → mediawiki.skinning.*

For consistency in module naming: skins.* contains the modules for
particular skins, mediawiki.* contains the modules for core MediaWiki
itself. Clearly this belongs more to the latter than the former.
'skinning' is probably the best thing to name these to not sound silly
and avoid confusion with actual skins.

(The fact that there are files in skins/ that are not skin-related is
a remnant of bad decisions in the past. I want to clean that up as
well this year.)

Updated release notes and usages, kept old definitions for HTML
backwards-compatibility (for 30 days on WMF deployment).

I intend to backport this to 1.23 (sans the compatibility modules).
Otherwise the current module names become "public API" and we either
break compatibility later or support them forever.

Change-Id: I3947eaacd389eaba32f6fc8370395caec4aa1edb