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

Issue: greg-112

It has been decided that a sponsor at a unit should have access to all
roles at that unit. This includes all units accessible through
hierarchical access.

Resolves: GREG-150

Dates can now be changed after they have started and ended. This means
that there is no situation which needs disabling the input fields, and
disabling has been removed.

Start and end date can now be in the past. The following rules apply:
 - Start dates can be any date in the past, and no more into the future
   than the max days property of the role type.
 - End dates follow the same rules
 - End dates must be equal to or later than start date.

Notification publishing has been reviewed to ensure duplicate
notifications are not created when start or end date is today.

Resolve: GREG-148

- Combine the /invite and /invitelink endpoints into one
- Return error messages from the backend
- Handle errors in the frontend
- Lower number of queries ran when calling the userinfo endpoint

Used to "log" a user out in the manual invite flow

The Role model has a not null constraint on the sponsor field, so it is
impossible for a Role object to not have this attribute.

Serialization has been moved to serializer classes, sharing them when
sensible and Views have been converted to ViewSets to simplify code.

Separates the queryset and the serialization from each other.

The OU view deserves its own file, and the guest info is actually a
person view and should live with  the other person views.

The profile page of a guest now shows a verification button if the guest
has a passport or national identificaiton number that has not been
verified. Clicking the button shows a dialog, with a confirmation button
which triggers a PATCH request to the backend and reloads the page when
it returns.

Resolves: GREG-101

Does the same as before with less and more readable code.

Any user that does not have a GregUserProfile must be a superuser so
this is not a problem per se, but it should be handled. For now we
simply treat them as an anonymous user and deny access.

Resolves: GREG-116

Issue: GREG-121-FEIDE-EMAIL