Load hi-res icons for JS-enabled devices (regression)

Hi-res menu icons were never loaded resulting in blurry icons for all
modern phones because ".jsEnabled #mw-mf-menu-main li.icon-home a" is a
weaker rule than "#mw-mf-page-left #mw-mf-menu-main li.icon-home a"
(rule started with id has preference).

Also, move all icon rules to mainmenu.less and avoid nested id rules (id
are unique anyway).

Regression introduced in 149f609.

Change-Id: Ic6fd7585d25a051895e1eea26acb2f4934c97120
Merge "Load hi-res icons for JS-enabled devices (regression)"