Replace image-url asset helper instances

Replace instances with CSS url() function. See rails/dartsass-rails#18.

Co-Authored-By: Martin Jones <[email protected]>

Author: jon-kirwan

app/assets/stylesheets/application.scss

@@ -1,3 +1,6 @@
+$app-image-path: "/assets/static/";
+$app-font-path: "/assets/static/";
+
 $govuk-new-link-styles: true;
 
 @import "govuk_publishing_components/govuk_frontend_support";

app/assets/stylesheets/helpers/_draft.scss

@@ -1,5 +1,5 @@
 .draft main {
-  background-image: image-url("draft-watermark.png");
+  background-image: url("draft-watermark.png");
   background-repeat: repeat-y;
   background-position: 50% 0;
   background-size: contain;