Use ECS events + EventBridge to track task success/failure

[epicfaace]

This will be more robust than our current approach.

https://docs.aws.amazon.com/AmazonECS/latest/developerguide/ecs_cwe_events.html

[epicfaace]

It seems like serverless framework comes with first-class support for eventbridge, so we can just trigger a lambda function instead of having to use a webhook -- see https://www.serverless.com/framework/docs/providers/aws/events/event-bridge/.

This is probably our best option, as terraform doesn't support eventbridge (see hashicorp/terraform-provider-aws#9330).