Move skin support into $wgVisualEditorSupportedSkins

$supportedSkins is mostly an artefact of the currently-unrefactored init code that
will have a sub-class for each skin (so skin's maintainers can add VisualEditor
support without hacking around inside VisualEditor itself), but until then let's
split it out into a global configuration variable at least, so that they have a
hope.

Change-Id: I0742cea6469678c5599768340dfb1f6d2f4debf4
Merge "Move skin support into $wgVisualEditorSupportedSkins"