Choose hosts when creating or editing an event (incl. recurring events)

Add a way to choose hosts (and co-hosts) while creating or editing an event, including recurring events — instead of only being able to promote people afterward from the Attendees dialog. **Where:** Event creation and edit flows, and recurring-event creation/edit. **What we'd like:** During create/edit, pick who the hosts and co-hosts are (e.g. add them by name), so the hosting team is set up front rather than assigned after the event exists. Related: "No option to promote an attendee to Host (only Co-host)".