Summary:
- Fly flyctl cleanup which has been failing; deleting volumes before app no longer seems needed
- Move fly-deploy and add workflow scripts to make fly preview work in grist-core too
Test Plan: Tested that previews still work in this repo; and tested with a copy in grist-core.
Reviewers: jarek
Reviewed By: jarek
Subscribers: jarek
Differential Revision: https://phab.getgrist.com/D3928
