Feature/graphql scaffold

[XanderVertegaal]

Part of #75

Basic implementation of a GraphQL API using Graphene (Django) en Apollo (Angular).

Don't forget to install dependencies and run yarn codegen when running the application frontend.

This PR also adds a SourceType, a query sources and two mutations: DeleteSource and UpdateOrCreateSource, so there is something to test. Visit localhost:8000/api/graphql to test them out.

Still to do (in other PRs):

• Handle CSRF correctly. I don't have much experience with this; any ideas are welcome. (#82)
• Use the correct URLs on production. For now, the endpoints in codegen.ts and graphql.module.ts are hardcoded.
• Haul over some handy utils from DIAPP (PortalDev, also uses GraphQL) that make mutations and error returning less boilerplatey.
• Add types/queries/mutations for the other objects in our data model. (I'll work on this next.)

I'll focus on writing more types and queries after this PR so we can start using them in the frontend when the views/components are there.

[XanderVertegaal]

To address the main issue of authentication: you're right that the endpoint used to be reachable for everyone. To address this, I've added a simple Middleware (only for requests aimed at the GraphQL endpoint) that check if the user is authenticated or not. You can test this by visiting localhost:8000/api/graphql and querying sources both when you're logged out and when you're logged in. I've also added a test to makes sure this doesn't break. (Run with python manage.py test)