Allow creating and setting a previously unknown owner from AK editing form

• Add a new link to edit form leading to owner creation form and passing the primary key of the edited AK/wish to that view
• To realize that link, the corresponding field is excluded from the default form rendering and the rendering is triggered manually for that field (and the name field that should still appear on top)
• When user is successfully created, they is set as owner of an AK, the user is redirect to the detail page of that AK and a message is displayed
• Add corresponding test
• This implements #96 (closed) and #86 (closed)

Additionally: Implement minor improvement for testing framework & add two more tests