Clear pomodoro ringing animation when session stops (#112)

clearPomodoroTimer() only cancelled the setTimeout but left the
ringing CSS animation on the button, so the tomato kept bouncing
after stopping a session.

Author: gusaiani

static/app.js

@@ -757,6 +757,7 @@ pomodoroBtn.addEventListener('click', () => {
 
 function clearPomodoroTimer() {
   if (pomodoroTimer) { clearTimeout(pomodoroTimer); pomodoroTimer = null; }
+  pomodoroBtn.classList.remove('ringing');
 }
 
 let _audioCtx = null;