Update User Migration

I reviewed this and it's looking great.

I think we should address the TODO as part of this round of migration improvements. I commented in the MR about it.

Rebased. It looks good, however, as we'll need to run a full user update that will take days, I need to coordinate the deployment of this.

RTBC, with no changes needed for now, but not deployed yet. Thanks!

Reviewed and merged. We will need a follow up for "contributor_task", but it can be separate to this issue.

Automatically closed - issue fixed for 2 weeks with no activity.