Deployment Records Deletion

[jfromme]

Currently when an Application is deleted we delete the associated infrastructure as well as the application record in the Applications DynamoDB table. But we leave the application's deployment records in the Deployments DynamoDB table.

This PR fixes this by also deleting all deployment records associated with the deleted application.

It also uses Pusher to send a new event type, application_deletion_event, on the application's channel to notify the frontend of the application's deletion. This event looks like:

{
  "application_id" : string, the application uuid
  "time": string, the time of the deletion
  "source": string, source of event. Maybe useful in debugging?
}