feat: Add ability to enforce SSO

Closes #543, closes #545

client/src/containers/UsersModalContainer.js

@@ -6,10 +6,12 @@ import entryActions from '../entry-actions';
 import UsersModal from '../components/UsersModal';
 
 const mapStateToProps = (state) => {
+  const oidcConfig = selectors.selectOidcConfig(state);
   const users = selectors.selectUsersExceptCurrent(state);
 
   return {
     items: users,
+    canAdd: !oidcConfig || !oidcConfig.isEnforced,
   };
 };