Use REST framework for json export & separate django app

This MR depends on !270 (merged). Previous development was on felix_bonn/akplanning!27 (closed)

This MR adds the app AKSolverInterface and moves the whole "export to JSON" logic into it. Further, the approach to generate the JSON changes from the as_json_dict functions to serializers of the Django Rest Framework.

Lastly, a new API endpoint<event_slug>/api/solver-export is added to retrieve the exported JSON directly.