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

added 7 commits

• 096208fa...df3607c0 - 6 commits from branch master
• 17cbacd5 - Notify sponsors about ending roles

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• 75d7c2e1 - just in case

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mentioned in commit dca89da3