NXT batches for manual pushes

For GiveCampus integrations and API.

A new manual/deposit export run creates an NXT gift batch when it does not yet have a stored batch ID. A retry of an existing run can reuse that run's stored batch. The push action does not offer an arbitrary existing NXT batch picker. Distinguish a new export from a retry before sending; do not assume that each retry necessarily creates a new batch or that new pushes can target any preexisting batch.