Refine migration script

[tublitzed]

As a follow up to the initial migration script, there's a few issues that were mentioned in the pr that may need some investigation (thanks @pjenvey for pointing them out):

• (probably) handle users exceeding batch limit
• how to handle errors during a user migration. can the entire user's move be in one transaction (so if an error happens, we cleanly abort)? and if we abort a user's migration should we retry? or just report the user id that failed for fixing up later?

If we think of more, let's just add them to the list here; if this list gets too big we can break down into smaller tasks.