Add icon to external links

app/root.css

@@ -640,6 +640,16 @@ a:not(:hover, :focus-visible) {
   text-decoration: none;
 }
 
+.contents a[target='_blank'][href^='http']:not(.icon-link, .transparent-link) {
+  padding-right: 12px;
+  background: url('/assets/Icon_External_Link.svg') no-repeat top 3px right 1px / 10px 10px;
+}
+
+sup {
+  /* external link icon above looks best when line-height does not change on lines with footnotes */
+  line-height: 1;
+}
+
 hr {
   border: 1px solid var(--colors-cool-grey-200);
   border-bottom: 0;