Add paid events in the app

[punchagan]

It should be possible to mark events as paid, and to specify the person who paid for an event, and the amount. The dues could possibly be added to Splitwise if we have the API integration setup correctly. The ideal would be to send UPI payment requests, but getting API access for that is going to be a pain, I think.

[punchagan]

This has been implemented in 059f096 24cc873 and eb03793 by adding a way to allow creating Splitwise groups for any RSVP event. The expenses have to be manually added on the Splitwise group, but it seems like a reasonable workflow, without needing to reimplement the whole Splitwise UI ourselves.