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

In the way that the "Link existing booking" feature works right now, it's only possible to select bookings that take place strictly within the start and end date time of the event.

It is often the case that rooms need to be booked for an event before it officially starts or after it officially finishes. Reasons to link an earlier or later booking may range from installing audiovisual equipment from a few days before the event to catering bringing drinks and food an hour before or having cleaning staff put everything back in order the day after.

Describe the solution you'd like

It should be possible to link bookings that take place earlier or later than the start/end date time of the event.

Describe alternatives you've considered

The implementation of this possibility brings up a number of non-trivial considerations:

• How should the linkable bookings be rendered in the UI? Could they appear colored different for easily conveying that the booking is happening outside of the event hours?
• Should it be possible to link to an arbitrary booking no matter of earlier or later it takes place compared to the event? If so, I believe that the UI to link the booking should have a way to enable/disable showing earlier/later bookings.
• If it's not possible to link to an arbitrary booking, then, how early or how late should it be possible to link bookings? Is one day enough? Or should it be two? Or why making this decision at development time and not letting it be configurable? And, if it is configurable, should it be configurable globally by admins or could this be configurable by room managers?

Additional context

This feature has been requested by Perimeter Institute. /cc @alittle60