R1:249ee9f1048a

Migrate mailing lists to mailing list users

Summary:
Ref T8387. This migrates lists in the database to users, and replaces all subscriptions.

This won't update Herald rules or saved search queries, but they're presumably rare and infeasibly complex to migrate.

Test Plan: This migration is relatively re-runnable, so I ran it a bunch of times with different setups using `bin/storage adjust --apply`. It successfully migrated lists into users and replaced them in all the places they were subscribed.