(core) clean up a collection of small problems affecting grist-core

Summary:
 * Remove adjustSession hack, interfering with loading docs under saml.
 * Allow the anonymous user to receive an empty list of workspaces for
   the merged org.
 * Behave better on first page load when org is in path - this used to
   fail because of lack of cookie.  This is very visible in grist-core,
   as a failure to load localhost:8484 on first visit.
 * Mark cookie explicitly as SameSite=Lax to remove a warning in firefox.
 * Make errorPages available in grist-core.

This changes the default behavior of grist-core to now start off in
anonymous mode, with an explicit sign-in step available.  If SAML is not configured,
the sign-in operation will unconditionally sign the user in as a default
user, without any password check or other security.  The user email is
taken from GRIST_DEFAULT_EMAIL if set.  This is a significant change, but
makes anonymous mode available in grist-core (which is convenient
for testing) and makes behavior with and without SAML much more consistent.

Test Plan: updated test; manual (time to start adding grist-core tests though!)

Reviewers: dsagal

Reviewed By: dsagal

Differential Revision: https://phab.getgrist.com/D2980

app/server/lib/GristServer.ts

@@ -2,6 +2,7 @@ import { GristLoadConfig } from 'app/common/gristUrls';
 import { Document } from 'app/gen-server/entity/Document';
 import { Organization } from 'app/gen-server/entity/Organization';
 import { Workspace } from 'app/gen-server/entity/Workspace';
+import { HomeDBManager } from 'app/gen-server/lib/HomeDBManager';
 import * as Comm from 'app/server/lib/Comm';
 import { Hosts } from 'app/server/lib/extractOrg';
 import { ICreate } from 'app/server/lib/ICreate';
@@ -25,6 +26,9 @@ export interface GristServer {
   getPermitStore(): IPermitStore;
   getExternalPermitStore(): IPermitStore;
   getSessions(): Sessions;
+  getComm(): Comm;
+  getHosts(): Hosts;
+  getHomeDBManager(): HomeDBManager;
 }
 
 export interface GristLoginMiddleware {
@@ -33,5 +37,5 @@ export interface GristLoginMiddleware {
   getLogoutRedirectUrl(req: express.Request, nextUrl: URL): Promise<string>;
 
   // Returns arbitrary string for log.
-  addEndpoints(app: express.Express, comm: Comm, sessions: Sessions, hosts: Hosts): string;
+  addEndpoints(app: express.Express): Promise<string>;
 }