Marking a car will turn the car as available even when it is not

[TPH777]

When there are multiple transactions of the same car model, even with non-conflicting end date, marking one as completed will immediately mark the car as available, even when other transactions are not completed.

Hence, I suggest to automate marking a transaction as completed when today's date passes the end date, instead of letting user mark themselves. mark can be used for ending a transaction early, so the end date will just be carried forward to today's date.