Remove generated files from Git

[XanderVertegaal]

As per #105 we currently check in the GraphQL schema and generated types, rendering diffs on PRs less insightful.

The DIAPP recently ran into the same issue of not being able to build a frontend code generation needs a running backend server. @tymees found a solution that we can use as well: https://github.com/CentreForDigitalHumanities/DIAPP/pull/267

It involves generating a static schema with python manage.py graphql_schema and using that as the basis for codegen. No need to run a backend dev server.