Fixed updating single show instance problem where it wasn't fetching
the current instance
Merge branch '2.5.x' of github.com:sourcefabric/Airtime into 2.5.x
CC-5589: SQL error when adding/moving a scheduled item

Error was occuring when a moved item didn't exist in a linked show instance
Error was also occuring when trying to insert item into a schedule localtion
that does not exist in a linked instance.

Fixed by skipping the move action if the file does not exist in a linked instance,
cleaning up the position numbers after deleting schedule items, and by
setting the insert time to the show start time in a linked instance is empty