Migrate to GitHub Actions

[egillespie]

To reduce the number of vendors required to manage the Lansing Codes software and simplify access control to builds and deployments, let's migrate the deployment pipeline from CircleCI to GitHub Actions.

The existing CircleCI configuration can be found in .circleci/config.yml. The pipeline should include these steps:

1. For all branches, perform a build:
   1. Checkout code
   2. npm ci
   3. npm rebuild
   4. npm test
2. If the build succeeds on the main branch, deploy to Heroku

@egillespie can set any environment variables required for the Heroku deployment.