(core) add minimal support for activation keys

Summary: For grist-ee, expect an activation key in environment variable `GRIST_ACTIVATION` or in a file pointed to by `GRIST_ACTIVATION_FILE`. In absence of key, start a 30-day trial, during which a banner is shown. Once trial expires, installation goes into document-read-only mode.

Test Plan: added a test

Reviewers: dsagal

Reviewed By: dsagal

Subscribers: jarek

Differential Revision: https://phab.getgrist.com/D3426

app/server/lib/IBilling.ts

@@ -4,4 +4,5 @@ export interface IBilling {
   addEndpoints(app: express.Express): void;
   addEventHandlers(): void;
   addWebhooks(app: express.Express): void;
+  addMiddleware?(app: express.Express): Promise<void>;
 }