Switch the scripts auth code to using UNUSABLE_PASSWORD

[adehnert]

We have some issues that are ultimately caused by the scripts cert auth code using "ScriptsSSLAuth" as the password for users who auth using certs. Really, this should probably(?) be UNUSABLE_PASSWORD (from contrib/auth/hashers.py, in Django 1.4). We should change the auth code to set that as the password, and migrate current ScriptsSSLAuth passwords (and maybe empty string, see ASA-#133) to be UNUSABLE_PASSWORD instead.

See ​https://diswww.mit.edu/menelaus/asa-db/2131.

[adehnert]

I did some considering, so update the description and title to reflect the current state.

[adehnert]

See Remit-#38 (​http://remit.scripts.mit.edu/trac/ticket/38) for some history. It looks like the fix for that was always wrong. Oops.

[adehnert]

Fixed:

Author: Alex Dehnert <adehnert@mit.edu>
Date:   Mon Dec 24 03:28:00 2012 -0500

    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.