Make background header larger than the body width

This way the curve will still work as intended on very large screens.

We're not taking into account the padding of the <main> element in the
SDG goals index for simplicity, since adding it barely has any effect.

app/assets/stylesheets/layout.scss

@@ -2563,6 +2563,11 @@ table {
     height: $line-height * 2;
     background: $highlight;
     margin-top: rem-calc(-48);
+
+    @include breakpoint($global-width) {
+      margin-left: calc(#{$full-width-margin});
+      margin-right: calc(#{$full-width-margin});
+    }
   }
 
   .cards-container {