Create an alert dispatcher with cancellation

I'm thinking it may be easier (both from a testing perspective, and from a coding perspective) to instantiate the Dispatcher() object inside runserver.py() right where the configuration object has been loaded up. It can then be passed into this class as a parameter. This simplifies testing (simpler mocks), and means that we can deal with the alerts.Dispatcher() configuration at the runserver() level rather than in here.