(core) Add basic activation page to grist-ee

Summary:
Adds an activation page to grist-ee that currently shows activation status.

Follow-up diffs will introduce additional functionality, such as the ability to
enter activation keys directly from the activation page.

Test Plan: No grist-ee tests (yet).

Reviewers: paulfitz

Reviewed By: paulfitz

Subscribers: dsagal, paulfitz

Differential Revision: https://phab.getgrist.com/D3582

app/server/lib/IBilling.ts

@@ -1,9 +1,9 @@
 import * as express from 'express';
-import {GristServer} from 'app/server/lib/GristServer';
 
 export interface IBilling {
-  addEndpoints(app: express.Express, server: GristServer): void;
+  addEndpoints(app: express.Express): void;
   addEventHandlers(): void;
   addWebhooks(app: express.Express): void;
   addMiddleware?(app: express.Express): Promise<void>;
+  addPages(app: express.Express, middleware: express.RequestHandler[]): void;
 }