(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/server/lib/IBilling.ts

@@ -0,0 +1,7 @@
+import * as express from 'express';
+
+export interface IBilling {
+  addEndpoints(app: express.Express): void;
+  addEventHandlers(): void;
+  addWebhooks(app: express.Express): void;
+}