Introduce detail and edit URLs as properties in AK model

This way, the frontend views will be used when AKSubmission is installed and active, or otherwise the link to the built-in AK edit form in backend
This reduces the coupling between AKSubmission and other apps that previously needed that app
Use that new property in all places where previously an URL to the submission view was used (for consistency and to allow changes in one place in AKSubmission, too)
This implements #166