Improve Stopwatch behavior (#18930)

- Don't send empty stopwatch over and over again, only send once.
- Stop interval to update stopwatch's timer when there is no more stopwatch.

web_src/js/features/stopwatch.js

@@ -127,6 +127,10 @@ function updateStopwatchData(data) {
   const watch = data[0];
   const btnEl = $('.active-stopwatch-trigger');
   if (!watch) {
+    if (updateTimeInterval) {
+      clearInterval(updateTimeInterval);
+      updateTimeInterval = null;
+    }
     btnEl.addClass('hidden');
   } else {
     const {repo_owner_name, repo_name, issue_index, seconds} = watch;