Self invite as attendee

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

We are a small non-profit-organization. We really love the Nextcloud calendar, but there are two small drawbacks with the attendee feature.

When creating an appointment in a shared calendar it is not possible to

• add yourself (and only yourself) as a attendee and
• to only add other people and not be the organizer (or change it afterwards)

Why is this useful?

• Person A creates an appointment for B and C but will not take part. Person A will be the organizer and cannot change it to A.
• Person A creates an appointment in a shared calendar and wants to have it in their personal nextcloud calendar also. Person A can't add themself as an attendee.

Describe the solution you'd like

• You should be able to change the role from any attendee to organizer.
• You should be able to add yourself as an attendee in appointments in order to see it in your own personal nextcloud calendar.

Describe alternatives you've considered

The alternative is that these calendar entries are created by a dummy user, which is not great in terms of security or managebility.

Additional context

No response

Start by tracing the existing attendee and organizer flows for shared calendars and personal calendars, including how the calendar app handles roles and invitations. Done means a user can add themselves as an attendee and change an attendee to organizer, while preserving the two shared-calendar scenarios described in the issue.