accounts/deactivated: Show deactivated_redirect url if present

If a user visits a realm which has been deactivated and it's
deactivated_redirect field is set, we should have a message telling the
user that the realm has moved to the deactivated_redirect url.

templates/zerver/deactivated.html

@@ -19,6 +19,13 @@
                 <p>
                     {% trans %}
                     The organization you are trying to join, {{ deactivated_domain_name }}, has been deactivated.
+                    {% endtrans %}
+                    {% if deactivated_redirect %}
+                        {% trans %}
+                        It has moved to <a href="{{ deactivated_redirect }}">{{ deactivated_redirect }}</a>.
+                        {% endtrans %}
+                    {% endif %}
+                    {% trans %}
                     Please contact <a href="mailto:{{ support_email }}">{{ support_email }}</a> to reactivate
                     this group.
                     {% endtrans %}

zerver/tests/test_signup.py

@@ -171,6 +171,16 @@ class DeactivationNoticeTestCase(ZulipTestCase):
 
         result = self.client_get('/accounts/deactivated/')
         self.assertIn("Zulip Dev, has been deactivated.", result.content.decode())
+        self.assertNotIn("It has moved to", result.content.decode())
+
+    def test_deactivation_notice_when_deactivated_and_deactivated_redirect_is_set(self) -> None:
+        realm = get_realm("zulip")
+        realm.deactivated = True
+        realm.deactivated_redirect = "http://example.zulipchat.com"
+        realm.save(update_fields=["deactivated", "deactivated_redirect"])
+
+        result = self.client_get('/accounts/deactivated/')
+        self.assertIn('It has moved to <a href="http://example.zulipchat.com">http://example.zulipchat.com</a>.', result.content.decode())
 
 class AddNewUserHistoryTest(ZulipTestCase):
     def test_add_new_user_history_race(self) -> None:

zerver/views/auth.py

@@ -601,8 +601,11 @@ def redirect_to_misconfigured_ldap_notice(request: HttpResponse, error_type: int
 def show_deactivation_notice(request: HttpRequest) -> HttpResponse:
     realm = get_realm_from_request(request)
     if realm and realm.deactivated:
+        context = {"deactivated_domain_name": realm.name}
+        if realm.deactivated_redirect is not None:
+            context["deactivated_redirect"] = realm.deactivated_redirect
         return render(request, "zerver/deactivated.html",
-                      context={"deactivated_domain_name": realm.name})
+                      context=context)
 
     return HttpResponseRedirect(reverse('login_page'))