After discussion at Pycon, we're changing "real_name" to "display_name" across
the board.

Schema changes:
   - real_name -> display_name (mailinglist, user, address)

Interface changes:
 * `IMailingList.real_name` -> `IMailingList.display_name`
 * `IUser.real_name` -> `IUser.display_name`
 * `IAddress.real_name` -> `IAddress.display_name`