compose_banner: Add update_or_append_banner method in compose_banner.

`update_or_append_banner` abstracts the logic for not creating another compose
banner if one is already present in the DOM, it just replaces the content of the
old banner with the new one.

web/src/compose_validate.js

@@ -262,7 +262,11 @@ function show_wildcard_warnings(opts) {
         );
     } else {
         // if there is already a banner, replace it with the new one
-        opts.$banner_container.find(`.${CSS.escape(classname)}`).replaceWith(wildcard_template);
+        compose_banner.update_or_append_banner(
+            wildcard_template,
+            classname,
+            opts.$banner_container,
+        );
     }
 
     user_acknowledged_wildcard = false;