Enable external contributors to create previews

[SleepyLeslie]

This PR reorganizes previews so they can be made for PRs from outside contributors.
The idea is to build a Docker image on the pull_request trigger (doesn't require write access to target repo / read access to our secrets), and later deploy it directly to fly.io.

[SleepyLeslie]

To test this PR:

• Make your own fork of grist-core.
• In your fork, create a secret named FLY_API_TOKEN. Paste your fly.io API token there.
• Fork your grist-core fork and create a PR from there to your grist-core fork.
• Attach the preview label to your PR.