Add slack notifications for AWS code pipeline events #364

There are some code pipeline workflows that failed a couple weeks ago after a PR merge in the backend repo

These went unnoticed for a while until we checked for an unrelated issue

It would be nice to have a Slack notification whenever a workflow fails. It would be even better if we can open the door to general AWS-related notifications

Useful resources:

• SNS notification channels: https://docs.aws.amazon.com/sns/latest/dg/sns-create-topic.html
• Events bridge to link SNS notifications to the AWS chatbot: https://docs.aws.amazon.com/eventbridge/latest/userguide/eb-what-is.html
• AWS Chatbot: Sends notifications to slack channels https://aws.amazon.com/chatbot/