(core) Move Notifier to /ext

Summary:
This makes it possible to configure a SendGrid-based Notifier
instance via a JSON configuration file.

Test Plan: Tested manually.

Reviewers: alexmojaki

Reviewed By: alexmojaki

Subscribers: paulfitz

Differential Revision: https://phab.getgrist.com/D3432

app/server/lib/GristServer.ts

@@ -22,6 +22,7 @@ import * as express from 'express';
  */
 export interface GristServer {
   readonly create: ICreate;
+  settings?: Readonly<Record<string, unknown>>;
   getHost(): string;
   getHomeUrl(req: express.Request, relPath?: string): string;
   getHomeUrlByDocId(docId: string, relPath?: string): Promise<string>;