Allow the user to protect his mythweb install from unauthorized access

We don't offer the user to protect his MythWeb install with a password. This is insecure and a potential security risk.

We should ask the user if he wants to protect MythWeb with a password via debconf. Maybe we can implement a very simple user management interface, starting by letting the user choose between deleting all existing accounts (htdigest -c) or adding a new account. Make sure to tell the user he can always dpkg-reconfigure to set a new password.