Fix bug with unit widget editor.

There were actually two bugs with the same feature:
* removing the numeric coefficient to a unit would cause an uncaught
  exception because KAS entered a different (unexpected) parse mode
* the unit examples were updated with the previous props in
  `componentWillReceiveProps`

Test plan:
* toggle between "5 kg m / s^2" and "5x kg m / s^2", see the thing
  toggle between green check and red x as expected
* "kg m / s^2" doesn't change it away from a green check, so that might
  be considered a buglet, but there were no exceptions

Auditors: alex