Resolves: GREG-173

Issue: greg-105

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

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

Issue: GREG-121-FEIDE-EMAIL

Adding tests for a serializer prompted moving of view tests to their own
subdirectory.