Bootstrap 3 upgrade
  * spanN is now col-md-N, etc.
  * Adjust fixed heights to compensate for border-box
  * Adjust tons of margins, especially for mobile
  * Set 16px font size on html
  * Use $screen-xs / $screen-sm vars instead of '768'

Some line heights and margins are different now, but hopefully
not in any way that will make anyone sad.