Deployment updates

[microbit-matt-hillsdon]

Plan:

• Check review branch deploy.
• Update review CF and DNS.
  • Done, e.g. https://review-python-editor-v3.microbit.org/deployment/ works. See also notes below.
• Remove redundant environment variables from CircleCI
• Merge this PR (not much choice to check staging properly)
• Check staging deploy
• Update staging versioner beta
  • Done, now serving at https://stage-python.microbit.org/v/beta
  • More testing required tomorrow
• Tag a pre-release
• Check release deploy
• Try /v/3/ on staging versioner
• /v/beta on live versioner, replacing current beta
• Use /v/beta in micro:bit classroom
  • Done but not released, one to discuss on Monday
• Add notes to the clean-up task

All done except for the classroom release which is tracked elsewhere.

[github-actions]

Preview build will be at
https://review-python-editor-next.microbit.org/deployment/

[microbit-matt-hillsdon]

Review deployments are working at https://review-python-editor-v3.microbit.org/deployment/

Remaining review actions:

• The GH action that puts the link on PRs needs updating.
  • Update deployment docs + link on PRs. #988
• The old CNAME can be removed from the CF distribution
• The old review content has been copied over but needs its acl prodding to make it visible
• The old bucket can then be deleted

[microbit-matt-hillsdon]

The staging deployment isn't quite right. It's deployed to the correct path but with PUBLIC_URL unset. This is because it's only called for the review stage. Follow-up PR here: #987