Modaxes shouldn't expect rendered view to be returned if being dismissed

[rowanseymour]

There should be a way for the view to pass back a response that says "please close" without passing back a rendering of the form. Sometimes the rendered form ends up invalid and for split second before the modal disappears the user needs new validation errors.

[ericnewcomer]

This works today by way of the temba-success header of "hide"