R1:a4a1143b1888

Fix an issue where internal paging of notifications could fail if some notifications are not visible

Summary:
Ref T13266. See <https://discourse.phabricator-community.org/t/notification-page-throws-unrecoverable-fatal-error/2651/>.

The "notifications" query currently uses offset paging for no apparent reason (just a legacy issue?), so some of the paging code is only reachable internally.

- Stop it from using offset paging, since modern cursor paging is fine here (and Feed has used cursor paging for a…