Add controls to unlink booking occurrence from object

[OmeGak]

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

After #6114 and #6243, it is possible to have booking occurrences linked to objects (i.e. events, session blocks, contributions). It is not possible, however, to remove the link between occurrences and objects.

Describe the solution you'd like

There should be UI controls to unlink objects from booking occurrences. This controls should be present in, at least, the following two page views:

Event management > Room bookings

Event management > Room bookings > Link to existing booking

Describe alternatives you've considered

Additionally, it could be possible to remove the links between booking occurrence and objects from the booking details modal.

[ThiefMaster]

Just wondering, why show an unlink option on the "Link to existing booking" view/page?

[OmeGak]

Now that I think of it, the unlink option from that view would only be useful for linking/unlinking the current object. Rather than having a dialog asking you if you really want to link it, a link toggle would make the UX smoother. Unlinking the occurrence from other objects wouldn't make a lot of sense from this view.