Clearing for a number of services

If a service supports clearing, it should create a `receive_alert_clear` method which will then be called for alert payloads that have `clear:true`.

Unclear at the moment whether or not services like customer-io need the clearing capability --probably not.  The slack integration needs to be done, as well as the impending VictorOps.

The idea here will be to get this into production and testing under a feature-flag and then incrementally support clearing for any services which do not already have it and for which it is applicable.