CC-5630: Shows created on a DST border get scheduled an extra hour longer

The show start and end time need to be in the show's local timezone when adding
or subtracting time to the start and end time (in case of DST crossings). In this
case the end time was already converted to UTC when we tried to change the time.
Merge branch '2.5.x' of github.com:sourcefabric/Airtime into 2.5.x