(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/declarations.d.ts

@@ -0,0 +1,5 @@
+declare module "app/common/MemBuffer" {
+  const MemBuffer: any;
+  type MemBuffer = any;
+  export = MemBuffer;
+}