(core) more grist-core cleanup

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

stubs/app/server/server.ts

@@ -1,3 +1,7 @@
+/**
+ * Main entrypoint for grist-core server.
+ */
+
 import {updateDb} from 'app/server/lib/dbUtils';
 import {main as mergedServerMain} from 'app/server/mergedServerMain';
 import * as fse from 'fs-extra';