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 (closed)