Add conditional check for existance of :failed_attempts on user model in...

The app/views/cardboard/users/edit.html.slim references an assumed model attribute failed_attempts. The doc doesn't mention this as a required field on the user model, and my model did not have this field. This caused an error while attempting to render this view. Added to the slim template a conditional check to see if the user model responds to the failed_attempts method before attempting to render information about the attribute.