Upstream report: https://secure.phabricator.com/T9275
When a new calendar event is created at https://phabricator.wikimedia.org/calendar/event/create/, the Visibility by defaulkt is All Users. It should be Public instead, as we are doing with tasks.
Items are editable by its author by default, but in the case of events that default might make sense. If someone is creating non-personal events, they can set permissions accordingly.