Use jistsi for element call

hosts/vm/periodique/element.nix

@@ -20,6 +20,11 @@
               feature_group_calls = true;
               feature_element_call_video_rooms = true;
             };
+
+            # https://github.com/element-hq/element-web/blob/develop/docs/jitsi.md
+            "jitsi" = {
+              "preferred_domain" = "jitsi.crans.org";
+            };
           };
         };
       };