(core) Converting server-side Comm.js to typescript

Summary:
- Add app/common/CommTypes.ts to define types shared by client and server.
- Include @types/ws npm package

Test Plan: Intended to have no changes in behavior

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3467

app/server/lib/Sessions.ts

@@ -3,6 +3,7 @@ import {cookieName, SessionStore} from 'app/server/lib/gristSessions';
 import * as cookie from 'cookie';
 import * as cookieParser from 'cookie-parser';
 import {Request} from 'express';
+import {IncomingMessage} from 'http';
 
 /**
  *
@@ -74,14 +75,14 @@ export class Sessions {
   /**
    * Returns the sessionId from the signed grist cookie.
    */
-  public getSessionIdFromCookie(gristCookie: string) {
+  public getSessionIdFromCookie(gristCookie: string): string|false {
     return cookieParser.signedCookie(gristCookie, this._sessionSecret);
   }
 
   /**
    * Get the session id from the grist cookie.  Returns null if no cookie found.
    */
-  public getSessionIdFromRequest(req: Request): string|null {
+  public getSessionIdFromRequest(req: Request|IncomingMessage): string|null {
     if (req.headers.cookie) {
       const cookies = cookie.parse(req.headers.cookie);
       const sessionId = this.getSessionIdFromCookie(cookies[cookieName]);