Calendar resharing is allowed in the UI but not implemented properly in the backend

[st3iny]

Steps to reproduce

1. Ensure that two users A and B exist.
2. Be user A: Create a calendar and share it with edit permissions with user B.
3. Be user B: Open the calendar modal and reshare the calendar with someone else.

The same applies if user A shares a calendar to a group.

Expected behavior

It should work as I'm allowed to share.

Actual behaviour

The dav request fails and there is no indication of a failure.

POST 403 https://localhost/remote.php/dav/calendars/admin/reshare-test-direct_shared_by_user/
<?xml version="1.0" encoding="utf-8"?>
<d:error xmlns:d="DAV:" xmlns:s="http://sabredav.org/ns">
  <s:exception>Sabre\DAV\Exception\Forbidden</s:exception>
  <s:message/>
</d:error>

The backend reports the incoming share as reshareable which is not correct. Hence the sharing options are shown in the modal.

Calendar app version

main

Nextcloud version

master

Additional info

Possible solution: Hide the sharing dialogue for now.