Notify sponsors about ending roles

The setup has five parts:
- a new email template type
- a new django-q task for notifying
- a management command for scheduling the task periodically
- a new settings variable NOTIFIER_LIMIT for controlling the number of
  days into the future the notifier script should use for end date of
  roles.
- a new settings variable NOTIFIER_SCHEDULE_TYPE for controlling how
  often the notifier task should be scheduled when using the management
  command.

Resolves: GREG-162