Room booking: show concurrent pre-booking warning when accepting a pre-booking

[panagiotappl]

Currently, there is no warning when accepting a pre-booking that indicates that one or more concurrent pre-bookings will be rejected. The user can only know this information by checking the Bookings timeline.

This is even less distinct when the user has to deal with weekly/bi-weekly bookings, where they would have to check all the occurrences in the timeline.

There could be a warning in the pre-booking dialogue with links to the concurrent pre-bookings and/or a checkbox that confirms the pre-booking rejection, as suggested by a user.