Separate handling of core skins MonoBook and Vector

per changes
https://gerrit.wikimedia.org/r/#/c/135413/
https://gerrit.wikimedia.org/r/#/c/138795/

Change-Id: I985dca4c657a3b8e7973339a220d082e0a1e487b
Merge "Separate handling of core skins MonoBook and Vector"