Allow for SSO only registrations #10

I'm not sure if I just don't see it but it doesn't appear like there's a way to have a config where signins/signups are only available via SSO/OIDC. Currently it looks like you can register via SSO but you would still need an invite code to do it if invite codes are enabled. If invite codes are disabled then someone can still make an account via normal registration steps and can bypass using SSO.