composebox_typeahead: Don't propagate after handling tab.

After we set focus manually to the send button, `tab` propagation
should be stopped.

frontend_tests/node_tests/composebox_typeahead.js

@@ -1054,6 +1054,7 @@ test("initialize", ({override, mock_template}) => {
             id: "stream_message_recipient_stream",
         },
         preventDefault: noop,
+        stopPropagation: noop,
     };
 
     $("#stream_message_recipient_topic").data = () => ({typeahead: {shown: true}});

static/js/composebox_typeahead.js

@@ -226,6 +226,7 @@ function handle_keydown(e) {
                     // typing a next message!
                     $("#compose-send-button").trigger("focus");
                     e.preventDefault();
+                    e.stopPropagation();
                 }
             } else {
                 // Enter