[Feature Request] Let public calendars be shareable

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As mention within the public calendar PR here, it would be very useful to have the public calendar to be shareable.

Imagine the following use-case:

Having a calender which should be shared with other users. The other users have read/write access to the shared calender, meaning a group of users can edit one calender. This calender can also be a public calender.

A good example is having an event calender. Some people add/edit events within this calender and want to make the event calender public available to publicity.

Some early tests:

I tested the behavior of having a shared calendar which is a public calendar also, by reverting the following change.

Everything seems to work. Having multiple users. One user got a calender which is shared (read only or read/write) to the other users. This calendar is then changed to a public calendar. The other users can read or write the shared calender without any problem. Only one problem occurred. As the owner of the shared/public calendar, It wasn't possible to subscribe to its own calendar with write access.