R1:0815891e4263

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.