Switch to Github Actions for all deployments

[neillrobson]

Moving to Github Actions for two reasons:

• Integration with Github Pages is superior compared with CircleCI
  More generally, deployments to both GCS and Pages are listed on PRs without additional scripting
• Authorization for non-engineering contributors (e.g. Design team members) to run deployments is more straightforward

To Do

• Remove dead CircleCI code
• Remove pr-comment script (assuming it's no longer necessary)
• Protect main branch against the build job
• Automatic deploys to Github Pages on main branch