Add an option to disable email+password sign-up

Self-hosters want to set up a single organization with SSO and then disable email+password sign-up for new users. We should provide an option for disabling the sign-up functionality. This can be done via en environment variable.

E.g. `AUTH_EMAIL_PASSWORD_REGISTRATION=1` and `AUTH_EMAIL_PASSWORD_REGISTRATION=0`

Note: it should still be possible for organization admin users to log in with email and password - as these can currently not be SSO users.