password: detect enterprise account without heuristic

Right now we detect an enterprise account by the lack of an
ActUser object.  That can't work going forward though, since other
bits of the code need an ActUser object for enterprise accounts.

This commit adds the account mode to the driver and changes the
password page to look at the account mode directly.

https://bugzilla.gnome.org/show_bug.cgi?id=752980