CRM-15578 - crmMailing(AB) - Refine pageflow:

CRM-15578 - crmMailing(AB) - Refine pageflow:

 * "Submit" => leave('scheduled')
 * "Delete" => leave('unscheduled')
 * "Save" => updateUrl()
 * "Send Test" => updateUrl()

This commit also switches back to managing pageflow from the controller
(rather than calling .then(leave) in the .html).  At the moment, it seems
more useful to have all the different composition screens follow the same
pageflow (rather than allowing them to have different pageflows).