Move $domain->user data to own table

Currently, we save $domain->user data in the 'data' field of the {users} table. This forces limitations on Views integration and introduces a constraint.

Move this data to a {domain_editor} table so it can be JOINed properly.

{domain_editor}

uid => Integer
domain_id => Integer

These are both foreign keys, so the unique constraint is on uid-domain_id.

Then write an update function to move all this data.