Set Agora font stack to mw-ui-container in Vector

We moved the language links outside the new compact vertical form
layout, hence also need to apply the font stack to mw-ui-container when
using Vector skin.

Also
* a comment-only CSS change
* set relative_assets = true in Compass sourcefiles configuration

Backported from master for new opt-in Create account form.

Bug: 47187
Change-Id: Id064dc8ada5c43c33e17d1e66d327ba05e918657
(cherry picked from commit e345e5bbb61ceb7349607470b71a05d4fb6bdc93)
Merge "Set Agora font stack to mw-ui-container in Vector" into wmf/1.22wmf2