Email notifications for GroupMemberships approaching expiry

[cc-a]

As a group membership approaches its expiry we need to make sure that the user in question and the owner of the group have plenty of notice of this so they can take appropriate actions.

Suggest waiting for #115 to be merged as this refactors some email functionality.

We'll need:

• a standalone email function that takes a group membership function as an argument and sends an alert email (to both the group member and the group owner) detailing when the expiration will occur.
• a new setting that allows configuration of the how far from expiry notifications should be sent - e.g. it might be set to [1, 5, 30] to indicate that email notifications should be sent 1, 5, and 30 days before a membership expires.
• a task function that identifies memberships that needs to have a notification sent (using the new setting) and sends the emails with the above email function.
• a data migration that populates a Django Q schedule running the task function daily (see Group expire #111 for an example of this).