Summary:
Snapshots can now only be listed for users with non-nuanced access
(no access rules, or owners on docs with rules). If a snapshot URL
leaks, or is shared by a user who can list snapshots, that URL
behaves as before -- it gives access to the snapshot according
to access rules in that snapshot.
Test Plan: added test
Reviewers: georgegevoian, dsagal
Reviewed By: georgegevoian, dsagal
Subscribers: jarek
Differential Revision: https://phab.getgrist.com/D3698