(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

app/common/tsconfig.json

@@ -1,5 +1,9 @@
 {
   "extends": "../../buildtools/tsconfig-base.json",
+  "include": [
+    "**/*",
+    "../../stubs/app/common/**/*"
+  ],
   "references": [
     { "path": "../plugin" }
   ]

app/common/version.ts

@@ -1,3 +0,0 @@
-export const version = "1.0.2-dev";
-export const channel = "devtest";
-export const gitcommit = "e33c4e5aeM";