[FIX] website: properly start all dropped snippets' animations

When a snippet is dropped into a page, all its animations are started.
The problem was that all its children's animations have to be started
and that was not the case anymore since recent big website refactoring.

Fortunately, this allows to improve and optimize animations start
function: now all animations are (re)started for a portion of a
given DOM (default to #wrapwrap) while it was only possible to
restart animation of a given DOM *element* before.

addons/website/static/src/js/content/website_root.js

@@ -119,17 +119,20 @@ var WebsiteRoot = BodyManager.extend({
      * (@see Animation.selector).
      *
      * @param {boolean} [editableMode=false] - true if the page is in edition mode
-     * @param {jQuery} [$initTarget]
-     *        only initialize the animations whose `selector` matches the element
+     * @param {jQuery} [$from]
+     *        only initialize the animations whose `selector` matches the
+     *        element or one of its descendant (default to the wrapwrap element)
      * @returns {Deferred}
      */
-    _startAnimations: function (editableMode, $initTarget) {
+    _startAnimations: function (editableMode, $from) {
         var self = this;
         editableMode = editableMode || false;
-        var $wrapwrap = this.$('#wrapwrap');
+        if ($from === undefined) {
+            $from = this.$('#wrapwrap');
+        }
         var defs = _.map(sAnimation.registry, function (Animation) {
             var selector = Animation.prototype.selector || '';
-            var $target = $initTarget ? $initTarget.filter(selector) : $wrapwrap.find(selector).addBack(selector);
+            var $target = $from.find(selector).addBack(selector);
 
             var defs = _.map($target, function (el) {
                 var $snippet = $(el);