(core) add appsumo endpoints with stub implementations

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

app/gen-server/ApiServer.ts

@@ -70,7 +70,10 @@ export function addOrg(
   return dbManager.connection.transaction(async manager => {
     const user = await manager.findOne(User, userId);
     if (!user) { return handleDeletedUser(); }
-    const query = await dbManager.addOrg(user, props, false, true, manager);
+    const query = await dbManager.addOrg(user, props, {
+      setUserAsOwner: false,
+      useNewPlan: true
+    }, manager);
     if (query.status !== 200) { throw new ApiError(query.errMessage!, query.status); }
     return query.data!;
   });