correct URL pattern used in text length counter in WebUI (#4968)

app/javascript/mastodon/features/compose/util/counter.js

@@ -1,7 +1,9 @@
+import { urlRegex } from './url_regex';
+
 const urlPlaceholder = 'xxxxxxxxxxxxxxxxxxxxxxx';
 
 export function countableText(inputText) {
   return inputText
-    .replace(/https?:\/\/\S+/g, urlPlaceholder)
+    .replace(urlRegex, urlPlaceholder)
     .replace(/(?:^|[^\/\w])@(([a-z0-9_]+)@[a-z0-9\.\-]+)/ig, '@$2');
 };