Allow setting NOTIFICATION_EMAIL_TO from a deployment var

[jwalgran]

Overview

The NOTIFICATION_EMAIL_TO setting is the address to which notifications about API usage limits are sent. We add the ability to set the value via a terraform variable during provisioning.

Connects #1202

Demo

The ECS task properties after deploying to staging

Testing Instructions

• Visually inspect the code and verify that task in the staging cluster have the environment variable set,

Checklist

• fixup! commits have been squashed
• CI passes after rebase
• CHANGELOG.md updated with summary of features or fixes, following Keep a Changelog guidelines

[TaiWilkin]

Thanks for the assist in getting to the containers! This works well, and I can see the correct variable in ECS.

[jwalgran]

Thanks for the review.