Go mobile first in our use of media queries.

* Go mobile first. Doesn't matter if IE6 takes up full screen.
* Tweak styles for desktop/tablet view
* Use a variable to represent tablet/desktop (as used in MobileFrontend)
* Drop hacks that make Flow board overlap Vector menus in small screen

Change-Id: I49c861f93ad0c012daa52e935e649ca525e27f59