Fix an error when users receive notifications about objects they can no longer see

Summary:
Ref T13623. The change in D21577 could lead to a case where we try to access stories the user can't see.

Move the story-loading piece to "willFilterPage()" to make our way thorugh this.

Test Plan:

• Made FeedStory return nothing to simulate invisible notifications, loaded page.
  • Before: index access fatal.
  • After: clean "no notifications".
• Loaded notifications normally, saw normal notifications.

Maniphest Tasks: T13623

Differential Revision: https://secure.phabricator.com/D21603