(core) Remove LoginSession, which was mainly serving situations that are no longer used.

Summary:
In the past, Cognito sign-ins were intended to give authorization to some AWS
services (like SQS); various tokens were stored in the session for this
purpose. This is no longer used. Profiles from Cognito now serve a limited
purpose: first-time initialization of name and picture, and keeping track of
which login method was used. For these remaining needs, ScopedSession is
sufficient.

Test Plan:
Existing test pass. Tested manually that logins work with Google and
Email + Password. Tested manually that on a clean database, name and picture
are picked up from a Google Login.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2907

app/server/declarations.d.ts

@@ -4,7 +4,7 @@ declare module "app/server/lib/User";
 
 declare module "app/server/lib/Comm" {
   import {Client, ClientMethod} from "app/server/lib/Client";
-  import {LoginSession} from "app/server/lib/LoginSession";
+  import {ScopedSession} from "app/server/lib/BrowserSession";
   import * as http from "http";
 
   class Comm {
@@ -14,7 +14,7 @@ declare module "app/server/lib/Comm" {
     public setServerVersion(serverVersion: string|null): void;
     public setServerActivation(active: boolean): void;
     public getSessionIdFromCookie(gristSidCookie: string): string;
-    public getOrCreateSession(sessionId: string, req: any): LoginSession;
+    public getOrCreateSession(sessionId: string, req: any): ScopedSession;
     public registerMethods(methods: {[name: string]: ClientMethod}): void;
     public getClient(clientId: string): Client;
     public testServerShutdown(): Promise<void>;