Convert Nave from Django application to a Django App dependency

[kiivihal]

We have decided to invert the direction of the depenency between the custom implementations and the main Django functionality. Therefore, we are refactoring the nave repository to become a Django app and it will be distributed and versioned via PyPI.

[hanswesterbeek]

I think the test-nave-app you've been working on, which takes the nave-artifactk as a dependency, should also be packaged as a docker-image. That way we can use it in an end-2-end testsuite.