Feature/VBV-290 kn backend
* Add initial assignment completion model
* Add first version of `update_assignment_completion`
* Upgrade wagtail>=4 for new functions needed in assignment api
* Add API to update assignment user data
* Post API via assignment not learning_content
* Add GET api endpoints for AssignmentCompletion
* Add some initial assignment completion data
* Add admin view for AssignmentCompletion
Approved-by: Elia Bieri