auth: also use safe password hashing on Windows using bcrypt

For unknown reasons, Kallithea used different hashing algorithms on Windows and
Posix. Perhaps because problems with bcrypt on Windows in the past. That should
no longer be an issue, and it doesn't make sense to have different security
properties on the platforms.

While changing to bcrypt everywhere, also remain backwards compatible and
accept existing passwords hashed with sha256 - both on Windows (where it used
to be used), and elsewhere (in case a system has been migrated from Windows to
Unix).