[FIX] website: properly update parralax image

Fixed some issues with parralax image updates:

- Background updated without clicking
- Default wallpaper not coming back
- Background disappearring sometimes

addons/website/static/src/js/website.snippets.animation.js

@@ -328,7 +328,9 @@ animation.registry.parallax = animation.Class.extend({
         if (!this.$bg || !this.$bg.length) {
             this.$bg = this.$("> .s_parallax_bg");
             if (!this.$bg.length) {
-                this.$bg = $("<span/>", {"class": "s_parallax_bg"}).prependTo(this.$target);
+                this.$bg = $('<span/>', {
+                    class: 's_parallax_bg' + (this.$target.hasClass('oe_custom_bg') ? ' oe_custom_bg' : ''),
+                }).prependTo(this.$target);
             }
         }
         var urlTarget = this.$target.css("background-image");

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

@@ -342,7 +342,6 @@ options.registry.parallax = options.Class.extend({
     _refresh: function () {
         _.defer((function () {
             this.$target.data("snippet-view")._rebuild();
-            this._update_target_to_bg();
         }).bind(this));
     },
     _toggle_refresh_callback: function (on) {

addons/website/static/src/less/website.snippets.less

@@ -409,7 +409,6 @@ div.carousel {
         background-size: cover;
         background-attachment: scroll;
         pointer-events: none;
-        z-index: -1;
     }
     &.s_parallax_is_fixed > .s_parallax_bg {
         background-attachment: fixed;

addons/website/views/snippets.xml

@@ -528,7 +528,7 @@
 </template>
 
 <template id="s_parallax" name="Parallax">
-    <section class="parallax s_parallax oe_custom_bg oe_img_bg"
+    <section class="parallax s_parallax oe_img_bg"
         style="background-image: url(/web/image/website.s_background_image_04);"
         data-scroll-background-ratio="1.6"><div class="oe_structure"/></section>
 </template>