Move Vector and MonoBook to separate repositories

This is the final step of the process described at
<https://www.mediawiki.org/wiki/Separating_skins_from_core_MediaWiki>.

Corresponding changes: I84fcf7ce in Vector, I83b1efb4 in MonoBook.

Change-Id: Idfc38503d8397ffe26e139037082d648bc5bfc5c