fix: css-loader file resolution bug introduced by CRA v4

In recent commits, we upgraded our react-scripts version from 3.4.4
to 4.0.3. This is because CRA (create-react-app) v3 uses an outdated
version of eslint (facebook/create-react-app#8849). This introduced
a bug related to the css-loader library, which can no longer resolve
assets in the public folder:
- facebook/create-react-app#9870 (comment)
- webpack-contrib/css-loader#1136 (comment)

This commit fixes this bug by moving the referenced image to the
relevant sub-directory in the src directory.

src/styles/isomer-cms/pages/Media.module.scss

@@ -10,7 +10,7 @@
 }
 
 @mixin checkered-background-grid {
-  background-image: url('/img/transparent_bg.jpg');
+  background-image: url('../../img/transparent_bg.jpg');
   background-size: 20px 20px;
   background-repeat: repeat;
 }