Add UI and actions for RSVP

[friedger]

A user should see yes/no buttons on events where the user is listed as guest (even if the event is read-only).

If there is a fee attached to the RSVP payment UI should be shown (the payment UI is part of a different issue).

If the user clicks yes (and there are no fees) a ticket object is stored on the user's storage (file path tickets/<appdomain>/<event-uid>)
If the user clicks no and there is a ticket already the ticket is deleted.

The initial state of the user's RSVP is retrieved from redux (checking for an existing ticket)