Migration to remove old users without email or uid

Keeps users who have editions published/pending/archived.

Keep important bot users

Also removing users with nil permissions

There are still duplicate accounts - either from using personal and gov
emails or just plain dupilcates like Pete - org id and version being
different, perhaps we need to merge these users?
Merge pull request #121 from alphagov/remove-old-users

Migration to remove old users without email or uid