use Date.now() instead of new Date().getTime()

This is documented to be more performant.

The substitution was made on frontend code, too (i.e., the one in /static),
because Date.now() is supported since IE 9, and we are life supporting only
IE 11.

Commands:
  find . -name *.js | xargs sed --in-place "s/new Date().getTime()/Date.now()/g"
  find . -name *.js | xargs sed --in-place "s/(new Date()).getTime()/Date.now()/g"

Not done on jQuery.

src/static/js/ace2_inner.js

@@ -1071,7 +1071,7 @@ function Ace2Inner(){
 
   function now()
   {
-    return (new Date()).getTime();
+    return Date.now();
   }
 
   function newTimeLimit(ms)

src/static/js/pad.js

@@ -475,7 +475,7 @@ var pad = {
   },
   _afterHandshake: function()
   {
-    pad.clientTimeOffset = new Date().getTime() - clientVars.serverTimestamp;
+    pad.clientTimeOffset = Date.now() - clientVars.serverTimestamp;
     //initialize the chat
     chat.init(this);
     getParams();