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. Notifications should NOT be generated while an active assignment session is open. It would be a nightmare if each drag-and-drop kicked off an email.

One idea is to put a flag on the Activity to track notification status (i.e., "needs notification"). This flag is toggled to true each time the Activity is dragged-and-dropped. The Activity should record its original state, so that if a contact is dragged back to her original shift during the course of an assignment session, "needs notification" is flagged off. Any time a flag is true, the UI lights up a button to notify the admin. Clicking the button sends an email to the related contacts.