Set status of an event (e.g. as paid)

[EmDee]

Similar to #49, a participant should be able to mark events as done/paid, so they won't be included in the payday.

[EmDee]

'Paid' events shouldn't be included in the event listing on the homepage anymore, thus reducing the total debt amount as well. Maybe we should have an archive of 'paid' events. 'payday' events should still be listed, until paid.