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

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

Start dates can now be blank, check for correct sponsor has been moved
to the more appropriate validate method and we now check that the role
exists before doing anything in the serializer

If the user is neither a guest or a sponsor, generic text explaining
this is shown. If they are a guest, they are shown the data related to
them. If they are a sponsor, they are redirected to the /sponsor/ route.

Resolves: GREG-97

To prevent hammering on the invite endpoint we throttle requests to the
endpoint to 100/day per ip.

Resolves: GREG-84