Notify Volunteers of Shift Assignment

When a shift assignment is made, notify the assignee and include the activity details in the email.
Message template should be manageable through the core template interface.

Ideally, notifications should NOT be generated while an active assignment session is open. This could be a can of worms though. Need to keep track of assignments that haven't had a notification sent and remove queued notifications if an assignment is cancelled.
Need a way to remind of unsent notifications and to be able to kick them off when schedule is complete.