Appointments: create per group

[WilliamDEdwards]

Is your feature request related to a problem? Please describe.

At the moment, appointments are tied to a single user.

For public links, such as planning a sales demo, the user is irrelevant. As long as the appointment is planned with a user that belongs to a group (e.g. has access to the chosen calendar), and that user is available.

Describe the solution you'd like

It should be possible to create appointments on a global or per-calendar level, rather than for a specific user.

When creating an appointment via the public link, a user will be chosen based on availability.

Describe alternatives you've considered

Store all links for all users in some frontend, and redirect to the appointments link of a random user. This does not guarantee that the chosen user is available, though.

Additional context

No response