Ensure SSO works on admin level and change implementation to be an external plugin

[mariobehling]

The SSO is currently listed on an organiser level in the platform, e.g. here https://app-test.eventyay.com/talk/orga/organiser/eventorganiser/settings/

As we are integrating SSO for the entire platform instead of integrating it for an organiser only the SSO integration should be accessible for admins only. It should show up as a tab in https://app-test.eventyay.com/talk/orga/admin/

Please move this feature into a separate module / repository to work as a plugin.