notify organizer of volunteer updates

Whenever a volunteer joins an event or un-volunteers, I wanted the organizer to be notified as well. I basically just duplicated the function that optionally sends a volunteer their own notification, made it non-optional, changed the from/to info a bit, and updated the content of the message.

The attached patch applies to volunteer_timeslots.module. I could imagine this might be optional on a per-node basis, but I didn't code that because it's not really necessary for our use case. If there's interest, I'll develop that out too.