Room Booking: add optional message on pre-booking acceptance

[pferreir]

A user suggested that we allow a message to be provided upon acceptance of a booking, pretty much like when it is rejected. In this case, the message wouldn't be mandatory and would also be included in the acceptance e-mail.
@ThiefMaster suggested a button similar to what GitHub uses for merging options:

Underneath the "arrow" menu we could have an "Accept with message" option.