Public scoreboard stopwatch feature

#10

app/views/contests/_stopwatch_js.html.slim

@@ -0,0 +1,22 @@
+javascript:
+  startTime = #{@contest.start_time.present? ? @contest.start_time.to_i : "null"};
+  pauseTime = #{@contest.pause_time.present? ? @contest.pause_time.to_i : "null"};
+  function updateTime() {
+    const displayTimeEl = document.getElementById('display-time');
+    if (displayTimeEl) {
+      if (startTime) {
+        let s = Math.floor((Date.now() - 1000 * startTime) / 1000);
+        if (pauseTime) s = Math.floor(pauseTime - startTime);
+        let ss = s % 60;
+        let mm = Math.floor(s / 60) % 60;
+        let hh = Math.floor(s / 3600);
+        displayTimeEl.innerHTML = `${hh < 10 ? `0${hh}` : hh}:${mm < 10 ? `0${mm}` : mm}:${ss < 10 ? `0${ss}` : ss}`;
+        setTimeout(updateTime, 1000);
+      } else {
+        displayTimeEl.innerHTML = '00:00:00';
+      }
+    } else {
+      setTimeout(updateTime, 20);
+    }
+  }
+  setTimeout(updateTime, 1);