Try to detect iOS and *not* autoplay any videos

Technically the check for iOS from components is a side effect, because it's
neither in state nor props, but the user agent does not change, so I think
that's okay.

app/assets/javascripts/components/is_mobile.jsx

@@ -3,3 +3,9 @@ const LAYOUT_BREAKPOINT = 1024;
 export function isMobile(width) {
   return width <= LAYOUT_BREAKPOINT;
 };
+
+const iOS = /iPad|iPhone|iPod/.test(navigator.userAgent) && !window.MSStream;
+
+export function isIOS() {
+  return iOS;
+};