Preserve last-known-good preview site

[bollwyvl]

what kind of request is this?

• a preview site on ReadTheDocs
  • it only adds a jupyak_config.toml
• a jupyak code change
  • it adds new features, described below

for jupyak code changes

references

• fixes Include previous lite builds in case of recoverable failure #26

code changes

• change the name of the generated lite archive to include the branch, e.g. jupyak-16-jupyterlite.tgz
• if a site fails to build, but a previous archive is available, download and extract it from _static/last-known-good

user-facing changes

• failing PRs that worked at least once should see a sidebar
  • 

backwards-incompatible changes

• if a PR was previously setting jupyter_lite_config.json#/output_archive in a gist, it will be ignored

[github-actions]

👈 Try it on ReadTheDocs

[bollwyvl]

Kicking tires here:

• https://readthedocs.org/projects/jupyak/builds/22869192/
  • if that fails (boo), it will test if the legacy lite-jupyterlite.tgz gets downloaded.
  • if that passes (yay), we don't care about the legacy url,
    • and can (hopefully) fix
      • then break the build to see the feature work as intended