Merge branch 'sh-fix-cross-site-forgery-errors' into 'master'

Return a blank JSON response for a missing .js file to prevent Rails CSRF errors

Closes #40771

See merge request gitlab-org/gitlab-ce!16664

app/controllers/application_controller.rb

@@ -147,6 +147,8 @@ class ApplicationController < ActionController::Base
       format.html do
         render file: Rails.root.join("public", "404"), layout: false, status: "404"
       end
+      # Prevent the Rails CSRF protector from thinking a missing .js file is a JavaScript file
+      format.js { render json: '', status: :not_found, content_type: 'application/json' }
       format.any { head :not_found }
     end
   end