protect fragile animation-end callbacks

If a second ring came in, the old callbacks would clear. If this
happened after the first part of the animation (likely), the callback
counter would never hit its target.

static/main.html

@@ -168,6 +168,7 @@
             // Instead, we can temporarily override the animation property to 'none' then the
             // animation will restart when we re-inheirit from the stylesheet.
             let flashers = $('.flash').each((_, e) => {
+              if (e.style.animationPlayState === 'running') return;
               e.style.animation = '';
               e.style.animationPlayState = 'running';