(core) move home server into core

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

app/common/tsconfig.json

@@ -1,3 +1,6 @@
 {
   "extends": "../../buildtools/tsconfig-base.json",
+  "references": [
+    { "path": "../plugin" }
+  ]
 }