Change calendar owner via Web UI

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Is your feature request related to a problem? Please describe.
Given a user creates a calendar and shares this by link publicly. When the user account gets deleted, the calendar is lost and all people who subscribed to it won't get notified properly.

Describe the solution you'd like
Being able to transfer a calendar (= changing the owner of the calendar) analogous to files as described in the user manual via the Web UI.

Describe alternatives you've considered
A corresponding occ command has already been implemented as requested (see https://github.com/nextcloud/server/pull/5881). However, it is not reachable from the UI and therefore not applicable for shared hosters or managed Nextcloud instances.

Start by reviewing the existing occ calendar-ownership transfer command referenced through issue #5158 and pull request #5881, then trace how calendar ownership is exposed in the Web UI. Done means an administrator can transfer a calendar owner through the UI, with the existing command's behavior preserved and publicly shared calendars retained after the original account is deleted.