Username exists check respects the relative root URL

Closes #25548

app/assets/javascripts/username_validator.js.es6

@@ -77,7 +77,7 @@
         this.renderState();
         return $.ajax({
           type: 'GET',
-          url: `/users/${username}/exists`,
+          url: `${gon.relative_url_root}/users/${username}/exists`,
           dataType: 'json',
           success: (res) => this.setAvailabilityState(res.exists)
         });

changelogs/unreleased/username-exists-root.yml

+---
+title: Username exists check respects relative root path
+merge_request: 
+author: