Add password conversions per auth type

[adetwiler]

Default password encryption is the password encryption currently used by the auth type. (i.e., sha1 or md5)
Preferred password encryption is what the passwords will be converted to if not using this encryption type.

[thomshouse]

Implemented this in a slightly different way than originally discussed. Escher will now use the best-possible encryption for passwords. Encryption is automatically upgraded per user on login.