Save DepositRequest record before scheduling loading

_complete_deposit() schedules a loading task; but it was called before the
DepositRequest object was saved to the database.

This means that if the loading tasks executes very fast, or the DepositRequest
takes a while to save, then the loader won't see the current DepositRequest when
it queries the deposit's current state.

This is especially likely to happen with large code uploads, as saving them to
disk happens before the DepositRequest row is written to the database.