Make it possible to unmount a series via admin UI

[LukasKalbertodt]

No description provided.

[oas777]

Would this be a solution for https://gitlab.elan-ev.de/opencast/eth/tobira/-/issues/29?

[wsmirnow]

This would be a part of the ticket. Deleting a series, without unmounting it first, should also result in "automatic unmounting" process.

[owi92]

#1225 added an unmount-series endpoint in Tobira. On that foundation it should be possible to add a corresponding endpoint in Opencast that can be called from the admin UI.

[owi92]

However, that endpoint only allows unmounting when the series is the only content on its page. We might need to make that more lenient and also add some function to delete any series block of that series. #1176 discusses this in detail.

[oas777]

Thanks, Ole. Question is whether/how @JulianKniephoff interpreted this in opencast/opencast-admin-interface#311.