Add ability to toggle Hotline members' notifications on/off

[raymondberg]

If someone is going out of town or is unavailable, being able to turn off their notifications with a radio button / switch is preferable to removing and re-adding later. Primarily because of the cost to re-add them to the system (2 sends, 1 receive per switch), but also because it means that the person adding them back to the roster needs to have/lookup their phone number and do manual reentry.