Set a password of UNUSABLE_PASSWORD

* Changes ScriptsRemoteUserBackend's configure_user method to set the default
  password to UNUSABLE_PASSWORD instead of ScriptsSSLAuth. UNUSABLE_PASSWORD
  displays in the admin as "Password: None", instead of an ugly error message.
  This should fix "Unknown password hashing algorithm" errors for users
  correctly created in the future. (ASA-#132)

* Adds a migration to change current users with passwords of "" or
  "ScriptsSSLAuth" to a password of UNUSABLE_PASSWORD ("!"). This will fix
  ASA-#132 and the symptoms of ASA-#133 for already-existent users.