Car-rental pop-up warning

This feature was proposed by the current car supervisor.

Car reservations made for friday-sunday aren't automatically confirmed unlike ones made for monday-thursday. These have to be confirmed (or denied) by the car supervisor before the renter can expect to have the car for those days.

To avoid oblivion, there should be a pop-up warning stating this fact, and asking the user to contact the car supervisor (bil@fsektionen.se) for a confirmation when the start/end date is a friday-sunday.