added guardian letters with chat and multiple answer functionalities

test/access/access_requirement_test.dart

@@ -0,0 +1,31 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:marianum_mobile/access/access_requirement.dart';
+import 'package:marianum_mobile/session/session.dart';
+
+void main() {
+  final student = CredentialSession(username: 'max', password: 'pw');
+  const guardian = GuardianSession(email: 'e@x.de');
+
+  test('nextcloud is only met by sessions with a Nextcloud identity', () {
+    expect(AccessRequirement.nextcloud.isMetBy(student), isTrue);
+    expect(AccessRequirement.nextcloud.isMetBy(guardian), isFalse);
+    expect(AccessRequirement.nextcloud.isMetBy(null), isFalse);
+  });
+
+  test('guardian is only met by guardian sessions', () {
+    expect(AccessRequirement.guardian.isMetBy(guardian), isTrue);
+    expect(AccessRequirement.guardian.isMetBy(student), isFalse);
+    expect(AccessRequirement.guardian.isMetBy(null), isFalse);
+  });
+
+  test('a set is met only when every requirement is met', () {
+    expect(<AccessRequirement>{}.areMetBy(null), isTrue);
+    expect(
+      {
+        AccessRequirement.guardian,
+        AccessRequirement.nextcloud,
+      }.areMetBy(guardian),
+      isFalse,
+    );
+  });
+}