Add support for Graphene-Django

In addition to the REST API support, support for GraphQL would be good.

1. Define queries and mutations for the [GraphQL schema](https://docs.graphene-python.org/projects/django/en/latest/schema/) in `api/schema.py`
2. [Re-use the REST API serializers](https://graphene-django-murali.readthedocs.io/en/latest/rest-framework.html)
3. Make sure the GraphQL queris and mutations use the same permission checks as the REST API viewsets do
4. Add documentation on how to inject the queries and mutations into the project schema