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/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;