Use REST framework for json export & separate django app

Addresses #26 (closed).

This MR does three things

1. Moves the logic into a separate django app (AKSolverInterface)
2. moves the logic of the as_json_dict functions into serializers of the Django Rest Framework
3. adds the API endpoint <event_slug>/api/solver-export to retrieve the exported JSON directly

• Allow POSTing of solver result to API