Storybook: Fix CSS url() assets

What does this MR do and why?

Resolves the CSS url() assets instead of always rendering a 1x1 Pixel transparent placeholder png.

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Screenshots or screen recordings

Before	After

How to set up and validate locally

1. yarn storybook:start
2. Add an background-image: url(); or background-image: image-url() in CSS (or use my patch below)
3. http://localhost:9002/?path=/story/vue-shared-crud--default
4. The background images should be shown

Diff:

diff --git a/app/assets/stylesheets/framework/crud.scss b/app/assets/stylesheets/framework/crud.scss
index 7fff0bc7384e..1365ee574db4 100644
--- a/app/assets/stylesheets/framework/crud.scss
+++ b/app/assets/stylesheets/framework/crud.scss
@@ -6,6 +6,11 @@ $crud-header-min-height: px-to-rem(49px);
 
 .crud-header {
   min-height: $crud-header-min-height;
+  background: url('swipemode_sprites.gif');
+}
+
+.crud-body {
+  background: image-url('emoji.png');
 }
 
 .crud-count .gl-badge {

Related to #499697 (closed)