Merge branch 'fix-username-validator-endpoint' into 'master'

Fixes endpoint to username validator

Fixes failing specs on master. Introduced here https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/6328

See merge request !6951

app/assets/javascripts/username_validator.js.es6

@@ -76,7 +76,7 @@
         this.renderState();
         return $.ajax({
           type: 'GET',
-          url: `/u/${username}/exists`,
+          url: `/users/${username}/exists`,
           dataType: 'json',
           success: (res) => this.setAvailabilityState(res.exists)
         });