Split skins.common up into a proper trio of .elements, .content, and .interface modules.

Ideally it would be possible to add these as dependencies and also make them depend on each other
instead of partially duplicating the list of stylesheets, but ResourceLoader’s dependencies are
completely broken for skin stylesheets.

Followup to Ia193571afffff36ecb1f14ee9036fb9bb98347e7

Change-Id: I56758908c2f41430c7301a0ea6a4cf8b1eaa7fd8
Release notes for Ia193571a, I56758908, I9fd40bcb

Change-Id: I6d7c4ba3592c5dd646180189086fd854ee6516a8
Merge "Split skins.common up into a proper trio of .elements, .content, and .interface modules."
Merge "Release notes for Ia193571a, I56758908, I9fd40bcb"