Fix an issue where paginating notifications could fail a GROUP BY test

Summary:
Ref T13623. When paginating notifications, we may currently construct a query which:

• loads from non-unique rows; and
• returns multiple results.

In particular, chronologicalKey isn't unique across the whole table (only for a given viewer). We can get away with this because no user-facing view of notifications is truly "every notification for every viewer" today.

One fix would be to implicitly force the paging query to include withUserPHIDs(viewerPHID), but puruse a slightly more general fix:

• Load only unique stories.
• Explictly limit the pagination subquery to one result.

Test Plan:

• Set page size to 1, inserted duplicate notifications of all stories for another user, clicked "Next", got the GROUP BY error.
• Applied the "only load unique stories" part of the change, got a "expected one row" error instead.
• Applied the "limit 1" part of the change, got a second page of notifications.

Maniphest Tasks: T13623

Differential Revision: https://secure.phabricator.com/D21577