Use named constants for magic numbers (#19992)

Mostly KeyboardEvent.which and MouseEvent.which values.

[skip validator]

js/src/util.js

@@ -16,6 +16,8 @@ const Util = (($) => {
 
   let transition = false
 
+  const MAX_UID = 1000000
+
   const TransitionEndEvent = {
     WebkitTransition : 'webkitTransitionEnd',
     MozTransition    : 'transitionend',
@@ -100,7 +102,7 @@ const Util = (($) => {
     getUID(prefix) {
       do {
         /* eslint-disable no-bitwise */
-        prefix += ~~(Math.random() * 1000000) // "~~" acts like a faster Math.floor() here
+        prefix += ~~(Math.random() * MAX_UID) // "~~" acts like a faster Math.floor() here
         /* eslint-enable no-bitwise */
       } while (document.getElementById(prefix))
       return prefix