implement friend quota ui + misc. improvements

- disable add friends widget when model.remainingFriendingQuota == 0
- show user the remainingFriendingQuota count
- use clearer spelling of condition determining when to show spinner
  and hide controls while waiting for model.friends to be populated
- hide current and suggested friends counts and only expand search
  box when in use to reduce clutter

closes getlantern/lantern#1283