[Feature] add Setting to allow Admins to create bookings fully in the past

[T3rminat0r]

Currently, both ways to create new bookings block admins from saving bookings that are completely in the past (Admins CAN save bookings with the enter date in the past (which is good, IMHO(!), don't "fix" this, please! :)). There is cases, how ever, where it'd be advantagous to be able to do just that. (I.e. when fixing booking errors by users, while maintaining an actually accurate booking history (User booked ONE continous booking from Monday to Friday, instead of one attendance each day...)

Putting this behind a seperate setting, so each site's Admins can decide wether they want/need this, or rather want to not be able to change bookings that are wholy in the past, at all.