Custom User Data

@esoergel @twymer
This is a fair test candidate for a "non-essential" db paradigm.  I've got no objections to doing that.  This doc type is probably O(domains), <10 per domain (and only advanced domains, at that), so I really can't see it getting big.  Makes sense to group this and any other "per-domain" configuration type things in another db from constantly-updating big doc types.