Include users role in JSON web tokens

The users role (e.g. `attendee` or `poster`) should be included in the JSON web tokens sent from the back end.

This will simplify things on the front end, and allow easily sending the role to the NextJS middleware so that it can be used to determine route access permissions.

Once completed, the global JwtPayload type (found in globals.d.ts) should be updated to include the field, and the value should be exposed from the access token in the AuthProvider.

Should be very simple: https://django-rest-framework-simplejwt.readthedocs.io/en/latest/customizing_token_claims.html