- remove is_primary

- remove is_primary
- set max on key length
- validate key_type against KeyTypes
- fix blacklisted key check (check for is_blacklisted IS True)
- accounts can have multiple keys but not vice-versa; move
  add_person/remove_person to Persons.add_key/remove_key
- validate SSH version 2 public keys
- add blacklist() function to permanently blacklist key (disassociate it
  and all other identical keys, and set is_blacklisted to True)
- whitespace nits
- Keys: only return non-blacklisted keys