(core) Send emails when 2FA settings are updated

Summary: When user 2FA status is changed, we now send out emails via SendGrid.

Test Plan: Server tests.

Reviewers: alexmojaki

Reviewed By: alexmojaki

Subscribers: alexmojaki

Differential Revision: https://phab.getgrist.com/D3280

app/server/lib/GristServer.ts

@@ -9,6 +9,7 @@ import * as Comm from 'app/server/lib/Comm';
 import { Hosts } from 'app/server/lib/extractOrg';
 import { ICreate } from 'app/server/lib/ICreate';
 import { IDocStorageManager } from 'app/server/lib/IDocStorageManager';
+import { INotifier } from 'app/server/lib/INotifier';
 import { IPermitStore } from 'app/server/lib/Permit';
 import { Sessions } from 'app/server/lib/Sessions';
 import * as express from 'express';
@@ -34,6 +35,7 @@ export interface GristServer {
   getHosts(): Hosts;
   getHomeDBManager(): HomeDBManager;
   getStorageManager(): IDocStorageManager;
+  getNotifier(): INotifier;
 }
 
 export interface GristLoginSystem {