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
