[FIX]Prevent displaying the users list in a meaningless sort order for end users in cases when usernames correspond to some numeric id's (ldap or similar), which are unrelated to Real Names. Cheap fix by means of allowing to sort by email, which is usually way more related to the real name than that numeric id. Better fix would have been to allow sorting by Real name, but that field is not found in users_users mysql table, so that it would have been more complicated to implement (and beyond my current skills).