use jQuery.promise to trigger callbacks instead of setTimeout

this makes sure callbacks for animations truly are called only
after the animation has finished. before, it'd be possible to trigger slightly ahead because of setTimeout tomfoolery

resources/static/pages/js/page_helpers.js

@@ -73,26 +73,19 @@ BrowserID.PageHelpers = (function() {
 
   function replaceFormWithNotice(selector, onComplete) {
     $("form").hide();
-    $(selector).fadeIn(ANIMATION_SPEED);
-    // If there is more than one .forminputs, the onComplete callback is called
-    // multiple times, we only want once.
-    onComplete && setTimeout(onComplete, ANIMATION_SPEED);
+    $(selector).fadeIn(ANIMATION_SPEED).promise().done(onComplete);
   }
 
   function replaceInputsWithNotice(selector, onComplete) {
     $('.forminputs').hide();
-    $(selector).stop().hide().css({opacity:1}).fadeIn(ANIMATION_SPEED);
-    // If there is more than one .forminputs, the onComplete callback is called
-    // multiple times, we only want once.
-    onComplete && setTimeout(onComplete, ANIMATION_SPEED);
+    $(selector).stop().hide().css({opacity:1}).fadeIn(ANIMATION_SPEED)
+      .promise().done(onComplete);
   }
 
   function showInputs(onComplete) {
     $('.notification').hide();
-    $('.forminputs').stop().hide().css({opacity:1}).fadeIn(ANIMATION_SPEED);
-    // If there is more than one .forminputs, the onComplete callback is called
-    // multiple times, we only want once.
-    onComplete && setTimeout(onComplete, ANIMATION_SPEED);
+    $('.forminputs').stop().hide().css({opacity:1}).fadeIn(ANIMATION_SPEED)
+      .promise().done(onComplete);
   }
 
   function emailSent(onComplete) {