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).
I've dug around a bit and we have ``TIME_ZONE = 'UTC'`` which according to https://docs.djangoproject.com/en/1.7/ref/settings/#std:setting-TIME_ZONE should mean that we store all datetimes as UTC regardless and thus that removing the TZ info here should be safe.