Simplify cancellation of bookings for ongoing events

[wiene]

Is your feature request related to a problem? Please describe.
It occasionally happens that events do not take place in a booked room and this only becomes clear around the start time of the event, e. g. because of a change of plans at short notice, because of an issue with the room equipment, because the room capacity turned out to be unsuitable, and what have you. In particular for longer lasting events or in case room occupancy is analyzed based on the booking data it is desirable to cancel the booking of the unused room. Unfortunately this cannot be done in a straightforward way after the scheduled start time has passed. In the situations described above this is often the case.

Describe the solution you'd like
Add a possibility to easily cancel bookings for ongoing events. At the moment this is only possible for future events, i. e. events whose start time is in the future. For reoccurring events the start time of today's instance needs to be in the future. The request only refers to ongoing events but not past events, i. e. those events whose end time has passed.

If this is not the desired behaviour for all Indico instances, one might consider making this feature configurable.

Describe alternatives you've considered
A possible workaround is to modify the event, move its start time to the future and cancel it. This approach is particularly cumbersome in case of reoccurring bookings where one needs to modify the start time of the reoccurring booking, cancel today's instance and move the start time back to the old value.