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