Summary:
* Remove duplicate schema file
* Move version file to a stub in grist-core
* Simplify sandbox creation in grist-core (although not functional until sandbox code moved)
* Add a minimal test for buildability
Test Plan: added test
Reviewers: dsagal
Reviewed By: dsagal
Differential Revision: https://phab.getgrist.com/D2560
Summary: This moves enough server material into core to run a home server. The data engine is not yet incorporated (though in manual testing it works when ported).
Test Plan: existing tests pass
Reviewers: dsagal
Reviewed By: dsagal
Differential Revision: https://phab.getgrist.com/D2552
