Export and Import Seating Plan as JSON

[mialbu]

A host should be able to export and import a json of a seating plan. To avoid a host creating the exact same seating plan every time a new event is planned, the host should be able to export the seating plan in a json. The next time an event is created, the host can just import this json file again, set the prices etc. for each type and then create the types without creating the whole plan from scratch.