Replace image-url asset helper instances

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

Author: jon-kirwan

app/assets/stylesheets/views/_bunting.scss

@@ -18,7 +18,7 @@
 }
 
 .app-bunting__triangles {
-  background-image: image-url("topical-events/bunting-grey-string.svg");
+  background-image: url("topical-events/bunting-grey-string.svg");
   background-color: transparent;
   background-repeat: repeat-x;
   background-position: center center;

app/assets/stylesheets/views/_organisation.scss

@@ -66,7 +66,7 @@
 .organisation__no10-banner {
   @include govuk-font(19);
   padding-top: 140px;
-  background-image: image-url("prime-ministers-office-header.jpg");
+  background-image: url("prime-ministers-office-header.jpg");
   background-repeat: no-repeat;
   background-position: 50% 0;
   text-align: center;

app/assets/stylesheets/views/_topical_events.scss

@@ -34,59 +34,59 @@
       }
 
       &.arts-council-england a {
-        background-image: image-url("topical-events/arts-council-england.png");
+        background-image: url("topical-events/arts-council-england.png");
         width: 217px;
       }
 
       &.bbc a {
-        background-image: image-url("topical-events/bbc.png");
+        background-image: url("topical-events/bbc.png");
         width: 143px;
       }
 
       &.british-library a {
-        background-image: image-url("topical-events/british-library.png");
+        background-image: url("topical-events/british-library.png");
         width: 30px;
       }
 
       &.commonwealth-war-graves-commission a {
-        background-image: image-url("topical-events/cwgc.png");
+        background-image: url("topical-events/cwgc.png");
         width: 209px;
       }
 
       &.heritage-lottery-fund a {
-        background-image: image-url("topical-events/hlf.png");
+        background-image: url("topical-events/hlf.png");
         width: 74px;
       }
 
       &.historic-england a {
-        background-image: image-url("topical-events/historic-england.png");
+        background-image: url("topical-events/historic-england.png");
         width: 242px;
       }
 
       &.imperial-war-museums a {
-        background-image: image-url("topical-events/iwm.png");
+        background-image: url("topical-events/iwm.png");
         width: 80px;
       }
 
       &.war-memorials-trust a {
-        background-image: image-url("topical-events/wmt.png");
+        background-image: url("topical-events/wmt.png");
         width: 103px;
       }
 
       &.birmingham_2022_commonwealth_games a {
-        background-image: image-url("topical-events/birmingham_2022.png");
+        background-image: url("topical-events/birmingham_2022.png");
         width: 120px;
         height: 120px;
       }
 
       &.queens_platinum_jubilee a {
-        background-image: image-url("topical-events/platinum_jubilee.png");
+        background-image: url("topical-events/platinum_jubilee.png");
         width: 120px;
         height: 120px;
       }
 
       &.unboxed a {
-        background-image: image-url("topical-events/unboxed.png");
+        background-image: url("topical-events/unboxed.png");
         width: 250px;
         height: 120px;
       }