Introduce the vector-body class
This follows up I4c1b15d90bacbc9b13782a1d8f52e838ce8ecd83
In that change, a new class for Vector specific styles should have
been added and the existing CSS referenced. I could have sworn I did
this, but obviously not (perhaps a git rebase or unstage change problem).
We did it for the other skins e.g. Monobook (I90d85c21f4a62e6697f24e3ce388445a0a53c2b0)
but evidently not Vector.
We also have to worry about cached HTML now, the #bodyContent is
the most reliable selector to use for before and after
Remove the mixin-clearfix rule on bodyContent - this is now
redundant with the changes in T279388 and should have been