Fix VE performance by cleaning up ui.less now universal header

A lot of the rules here apply to the old header on special pages,
these have now been removed.

Reshuffle the css and drop the use of the expensive nth child selectors
in the header so that VisualEditor performance will improve.

Bug: 64719
Change-Id: I8a26f0564d89aa9b988026d2c6384b58cb546b64