Heuristic for telling if guest is registered and verified

See merge request !26

GREG-17: Duplicate check

See merge request !21

GREG-21: Add foedselsnummer as type

See merge request !25

Adds a convenience method, is_verified() -> bool, to the Person model
that returns true if the guest has at least one identity verification method
associated with them.

It is the combination of is_verified() and is_registered() that qualifies
a guest for account creation in the IGA.

Fixes: GREG-20

Adds convenience method to Person for testing if the guest has
completed registration.

Fixes: GREG-20

A Person may have multiple associated PersonIdentities.  The object
relation link between these data structures was previously introduced
as "person", but is renamed "identities" with this patch.

This changes the access pattern from:

	person.person.all() -> QuerySet[PersonIdentity]

To:

	person.identities.all() -> QuerySet[PersonIdentity]

Fixes: GREG-20

Fixes: GREG-20

Adding first and last name to sponsor model

See merge request !22

Mostly a visual improvement, but instead of importing the entire
django_filters.rest_framework module we make the dependency situation
clearer by importing specific symbols.

Fixes: GREG-20

The list(self, request, *args, **kwargs) function drops args and kwargs
by not passing them on to the super class.

Fixes: GREG-20

GREG-17: Adding constraint to avoid duplicate roles

See merge request !23

Add sponsor guest endpoint

See merge request !20