Fix rendering of offset-paged query panels including "Notifications"

Summary:
See https://discourse.phabricator-community.org/t/call-to-undefined-method-phuipagerview-gethasmoreresults-in-2019-week-13/2586/.

A small number of queries (including "Notifications" and (global) "Search") use offset-based pagers which have a slightly different API PHUIPagerView instead of AphrontCursorPagerView. This leads to a fatal in the new code for the "View All Results" buttons.

To fix this, just do an instanceof test. Some day we can unify the pagers.

Test Plan: Added a notifications panel, rendered it, saw it work instead of fataling on "getHasMoreResults()". Also rendered some normal panels.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20366