Merge branch 'namespace-validation' into 'master'

Check all namespaces on validation of new username.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/24519 and https://gitlab.com/gitlab-org/gitlab-ce/issues/24580

See merge request !7537

app/controllers/users_controller.rb

@@ -86,7 +86,7 @@ class UsersController < ApplicationController
   end
 
   def exists
-    render json: { exists: Namespace.where(path: params[:username].downcase).any? }
+    render json: { exists: !!Namespace.find_by_path_or_name(params[:username]) }
   end
 
   private

changelogs/unreleased/namespace-validation.yml

+---
+title: Check all namespaces on validation of new username.
+merge_request: 7537
+author: