Summary:
This adds appsumo /token and /notification endpoints, with some
tests. The stub implementation is sufficient for AppSumo
activation to succeed (when exposed via port forwarding for testing).
It needs fleshing out:
* Implement upgrade/downgrade/refund and stripe subscription.
* Implement custom landing page and flow.
Test Plan: added tests
Reviewers: dsagal, georgegevoian
Reviewed By: dsagal
Subscribers: alexmojaki
Differential Revision: https://phab.getgrist.com/D2864
