Currently, events default to "private events", effectively: they are visible to the host and invitees.
I think we have enough distinct types of events to present a modal option prior to event creation. Basically, turn "Create Event" into a dropdown:
Create Private Event
Create Public Event
Schedule Meeting (T8022)
Create Recurring Event (T2896)
For "Public", the only difference for now is that "Visible To" defaults to "getMostOpenPolicy()".
For "Schedule Meeting", we'd set the isFluid flag and switch to constraint-based scheduling (T8022).
For "Recurring Event", we'd use an alternate UI we eventually build in T2896.