Private venues — add locations that aren't publicly listed

Add a way to create a **private venue** — a location that isn't publicly listed or searchable, for example someone's home or a members-only space. **Where:** Venue creation and the venue picker when creating an event. **What we'd like:** - Mark a venue as private when adding it - A private venue doesn't appear in the public venue list/search - Its address is only shown to the people who need it (e.g. the event's attendees or the group's members) **Why:** Hosts regularly meet at private homes or member-only locations and don't want that address publicly listed for everyone to browse.