Move mediawiki.skinning.* modules from skins/common/ to resources/

This is a BREAKING CHANGE, as we have used the names of these three
files in skins/common/ as a "public API" for skins for a long time.
Doing so has been deprecated (but still supported) in 1.23.

We must clear them out at some point, and now is as good a time as
ever, with the 1.23 LTS release still supporting the old way.

Skins may no longer rely on the presence of commonElements.css,
commonContent.css and commonInterface.css in skins/common/ and
include them in their style modules. The corresponding ResourceLoader
modules must be used instead:

- skins/common/commonElements.css  → 'mediawiki.skinning.elements' module
- skins/common/commonContent.css   → 'mediawiki.skinning.content' module
- skins/common/commonInterface.css → 'mediawiki.skinning.interface' module

Follow-up to I3947eaac (where I said I intend to do this) and I56758908.

Bug: 69277
Change-Id: Id8456c570ff406822a8befad8f42f528ef5bd3a6