Optimize timestamp formatting

Date.toLocaleDateString() with options is extremely slow,
creating and re-using an Intl.DateTimeFormat object is two
orders of magnitude faster (see #2820)

Fixes #2820

h/static/scripts/date-util.js

-var DATE_SUPPORTS_LOCALE_OPTS = (function () {
-  try {
-    // see http://mzl.la/1YlJJpQ
-    (new Date).toLocaleDateString('i');
-  } catch (e) {
-    if (e instanceof RangeError) {
-      return true;
-    }
-  }
-  return false;
-})();
+// cached date formatting instance.
+// See https://github.com/hypothesis/h/issues/2820#issuecomment-166285361
+var formatter;
 
 /**
  * Returns a standard human-readable representation
  * of a date and time.
  */
 function format(date) {
-  if (DATE_SUPPORTS_LOCALE_OPTS) {
-    return date.toLocaleDateString(undefined, {
-      year: 'numeric',
-      month: 'short',
-      day: '2-digit',
-      weekday: 'long',
-      hour: '2-digit',
-      minute: '2-digit',
-    });
+  if (typeof Intl !== 'undefined' && Intl.DateTimeFormat) {
+    if (!formatter) {
+      formatter = new Intl.DateTimeFormat(undefined, {
+        year: 'numeric',
+        month: 'short',
+        day: '2-digit',
+        weekday: 'long',
+        hour: '2-digit',
+        minute: '2-digit',
+      });
+    }
+    return formatter.format(date);
   } else {
     // IE < 11, Safari <= 9.0.
     // In English, this generates the string most similar to