Honeybadger integration

https://trello.com/c/aM9S3Dxm/1459-oct-15-forward-emails-error-events-from-customer-io-to-honeybadger

WIP: Notify honeybadger when email event_type is an error.

@KevinBongart Started this PR and need your help getting it across the finish line. I started a rake task to mimic the webhook locally for development, but the docs on Customer.io and the params I see logged in Heroku don't match my reading of the code for the format of the request. Your expertise is welcomed.