Migrate CI to AWS CodeBuild

[onyxfish]

This PR migrates the CI for this project to AWS CodeBuild. This has a few benefits:

1. The build now happens inside the AWS firewall, which will make it easier for us to configure automated deployments.
2. The build infrastructure is within the existing Sceptre configuration, which makes it wonderfully self-documenting.
3. Having all the configuration in one place will also make it easier to manage the permissions specifically required for deployments.

The AWS builds automatically report status back to GitHub and will block the build if they fail. They also include a link that goes directly to the build log in CodeBuild (example):

Notes

• This PR is stacked on Backport sceptre best practices for zephier-contrib-web #55.
• The CI workflow itself should be identical to what was previously running on GitHub Actions.
• The CodeQL scans were failing with an error. I tried everything I could think of to disable the CodeQL scanning entirely, but I simply can't get it to turn off. By adding a blank workflow file it seems to at least be passing again. 🤷

[cscollett]

lgtm. Maybe you can demo this through at pair programming