(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/gen-server/lib/HomeDBManager.ts

@@ -35,6 +35,7 @@ import {makeId} from 'app/server/lib/idUtils';
 import * as log from 'app/server/lib/log';
 import {Permit} from 'app/server/lib/Permit';
 import {WebHookSecret} from "app/server/lib/Triggers";
+import {StringUnion} from 'app/common/StringUnion';
 import {EventEmitter} from 'events';
 import flatten = require('lodash/flatten');
 import pick = require('lodash/pick');
@@ -48,6 +49,17 @@ import * as uuidv4 from "uuid/v4";
 // fixed.  See https://github.com/typeorm/typeorm/issues/1884#issuecomment-380767213
 applyPatch();
 
+export const NotifierEvents = StringUnion(
+  'addUser',
+  'userChange',
+  'firstLogin',
+  'addBillingManager',
+  'teamCreator',
+  'trialPeriodEndingSoon',
+);
+
+export type NotifierEvent = typeof NotifierEvents.type;
+
 // Nominal email address of a user who can view anything (for thumbnails).
 export const PREVIEWER_EMAIL = 'thumbnail@getgrist.com';
 
@@ -190,6 +202,10 @@ export class HomeDBManager extends EventEmitter {
 
   private _docAuthCache = new MapWithTTL<string, Promise<DocAuthResult>>(DOC_AUTH_CACHE_TTL);
 
+  public emit(event: NotifierEvent, ...args: any[]): boolean {
+    return super.emit(event, ...args);
+  }
+
   /**
    * Five aclRules, each with one group (with the names 'owners', 'editors', 'viewers',
    * 'guests', and 'members') are created by default on every new entity (Organization,