feat(event): add "unlisted" visibility

[dargmuesli]

Currently, events can be public or private which changes if they appear in search results. Private event show in overall search results only for people who organize the event or for guests who are invited. If someone, who hasn't been invited to an event (i.e. does not have access to the invitation link), opens the event link (which is not the same as invitation link), access to the event page is denied.

Now, there could be a new visibility: "unlisted", which hides an event from overall search results, but people can still share the event link and access the event page without having been invited.

Waiting for:

• feat(event)!: differentiate private and unlisted visibility sqitch#18