Only refresh the prompt cell when showPrompt is toggled in the MBContactCollectionView.  This provides a nicer user experience where cell highlight state is maintained and an animation is used for the addition/removal of the prompt.
If we have to use a magic constant, lets at least put it in one place.
Prompt cell required width calculation wasn’t taking the UILabel’s font into account so it wasn’t growing wide enough.  Fix.
Get rid of automatic collection view reload when changing prompt settings at the MBContactPicker level