Add seating information to ticket

[feliciaan]

For some events, when the user arrives a seat or a zone with seats is given.
It's different from access_level and because it can be added to different people with the same access_level.
This is not user facing information, as its only seen by admins.

[feliciaan]

To add seats we could import an excel sheet or csv with an seating location field.
This data will be fixed offline.