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, thanks for reminding me about this. Tests fail for me on SQLite if I try use timezone-aware datetimes as `from_date` or `to_date`. This affects `go_gen_statement` too. Do the `go_gen_statement` tests pass for you locally on SQLite? I'm not entirely sure if using non-aware datetimes is acceptable here -- I think it is, because we construct non-aware datetimes elsewhere and I think Django will treat those as UTC given our settings, but I should double-check.