Skin: Make normalizeKey() accept non-canonical case variants of keys

For example, passing 'MonoBook' and 'monobook' will now both yield
'monobook'; previously, the former yielded the default skin ('vector').

This magically fixes casing issues in the &useskin= URL parameter, as
well as in configuration variables like $wgDefaultSkin.

(This is per agreement on wikitech-l discussion a few weeks ago,
"What should be the recommended / supported way to do skins?").

Change-Id: I0df77b6d42726605b56d52b34e204113e8089e80
Merge "Skin: Make normalizeKey() accept non-canonical case variants of keys"