New design

https://www.mediawiki.org/wiki/Flow_Portal/Design/Iteration_5

Removing all existing CSS
Starting anew, from scratch, in LESS

modules/mediawiki.ui is new Agora, may make sense to some day merge into core
Other modules/* are Flow-specific

Change-Id: Ibb77ba1321ddaf4269bffa1f5881f679f7ebd9e3
Merge "New design"