Add terraform to subscribe to email delivery/failure events

[tdooner]

Ticket

N/A - Relates to NYC debugging.

Changes

This adds terraform code to get us most of the way towards processing the delivery status of every email: it sets up delivery event notifications for any successfully delivered email, or any email that is not deliverable due to bounce.

The remaining work is to actually subscribe the application to receive these events as HTTP webhooks, either directly via SNS or by creating a Lambda to send the events. Until that work is done, we can get visibility into email deliverability (for debugging purposes) by subscribing our personal email addresses to it. (Again, only for debugging purposes until we have actual real data.)

Context for reviewers

Here's what I'm seeing after sending three test invitations to various good/bad emails:

Testing

Tested by running this locally and creating all the resources in our demo environment.

[allthesignals]

@lorenyu any thoughts on this?

[tdooner]

It turns out this was unnecessary for NYC debugging, but we probably should still do it in tandem with handling these webhooks in Rails to track deliverability for product metrics reasons.

[allthesignals]

@tdooner Should we merge it? It'd be good to know when these fail

[tdooner]

@allthesignals Yeah I think merging it is fine, I don't think it's too risky. We would still have to write the actual handler to subscribe to receive these events.

[allthesignals]

@tdooner should we merge this? Been open for a while...