Clean up width toggle styles

Authored by Catrope.


Clean up width toggle styles

  • Move styles affecting .flow-board from widget styles to common.less so they load early enough, fixing a FOUC where the max-width of the board changed from 850px to 1200px
  • Remove unnecessary rules
    • display: block; clear: left; box-sizing: border-box; on .flow-board
    • display: block; float: right; set twice on expanded sidebar
  • Use @medium variable

Bug: T103584
Change-Id: I48aa80fafb92e331e2143d3f72eeca131b172a57