Mark passwords for SSO-only users as invalid to allow changing them later

Add a new "hash type" for invalid passwords, which is never equal to
normal passwords, but nevertheless can be changed without being known by
the user.

This "invalid" password can only be set by directly setting the password
hash type. When updating the password using update_password method, it will
always be upgrade it to the strongest hash type, sha256crypt.

To allow changing this "invalid" password to a normal one, stop
requiring a non-empty current password in the password change dialog
when changing a password from an "invalid" one. Don’t show the current
password box either, as it is not used anyway in this case, making
it better not to show it to avoid confusion.

Signed-off-by: Andrej Shadura <andrew.shadura@collabora.co.uk>
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