[FIX] website: bg element not found when using big parallax snippet

qsm:
This is because of multiple animations during edition (which is not
occuring afterwards). This is not supported, but as it does not break
anything, it can be kept as it is. However, we cannot access the
animation via 'snippet-view' anymore, which is fine as I am about to
deprecate it in master.

addons/website/static/src/js/editor/snippets.options.js

@@ -341,7 +341,7 @@ options.registry.parallax = options.Class.extend({
         this.trigger_up('option_update', {
             optionNames: ['background', 'background_position'],
             name: 'target',
-            data: this.$target.data('snippet-view').$bg,
+            data: this.$target.find('> .s_parallax_bg'),
         });
         this._refresh();
     },