Co-presenter handling

[flaki]

While Waasabi is perfectly fine with having multiple people involved with a session, the CFP app currently doesn't allow for co-presenters. At the baseline having a single main contact person is fine but we should allow specifying "other people involved", who would be contacted upon acceptance in the same way the main presenter is.

We would then go on and create a session for them in the same way as before, with multiple people hosting the session.