alerts: update model when cancellation unscheduled + doc for Alerter
making rest api suitable for script calls

* expose as /rest/do in addition to /console/do
* redirect and set message as cookie only if referer is set, otherwise
  (i.e. for scripts call) send 200 or 500 regarding message type (errors
  and warnings imply 500) and write message as a payload
* minor code simplification
adding a lacking qRegisterMetaType