Notification if external user management is activated

A notification will be displayed when an external user
management system is activated to inform that users
created within the SCMM will not be transferred to the
external system.

Co-authored-by: René Pfeuffer<rene.pfeuffer@cloudogu.com>

scm-ui/ui-api/src/users.test.ts

@@ -72,6 +72,7 @@ describe("Test user hooks", () => {
     _embedded: {
       users: [yoda],
     },
+    externalAuthenticationAvailable: false,
   };
 
   afterEach(() => {