Add management command for archiving transactions

Similar to the one in #1083 that handles statement generation.

This should support archiving to S3 but not deleting the transactions (for carefully trying things out in production).
Ah, I actually have only tested `go_gen_statement` with postgres, so I'll check with sqlite. Given we are okay with non-aware datetimes, I can change `go_gen_statement` to do the same thing.