Hygiene: Cleaning up getSkinConfigVariables and improving documentation

Also getting rid of wgUseFormatCookie since it's no longer used, and
removing the getSkinConfigMobileVariables function since it doesn't
need to be a separate function.

Change-Id: Id93eff1f7635be558024d1d8e04ef1e5af8c32d6
Merge "Hygiene: Cleaning up getSkinConfigVariables and improving documentation"