(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/mergedServerMain.ts

@@ -98,6 +98,7 @@ export async function main(port: number, serverTypes: ServerType[],
 
   server.addAccessMiddleware();
   server.addApiMiddleware();
+  await server.addBillingMiddleware();
 
   await server.start();