Raise PRs on repos that use this image

[symroe]

When a new image is made, there should be some way of raising a PR on the repos that use this image.

Ideally each project would be able to listen for new images, but that would add complexity to every project. Rather than that, and because there is a small number of projects who are all in our control, we should push new changes from this project.

This looks like a Lambda that listens for new images (via SNS) and raises the PR / issue on each project.