R1:773b4eaa9ea0

Separate "feed" and "notifications" better, allow stories to appear in notifications only

Summary:
Depends on D19861. Ref T13222. See PHI996. Fixes T10743. Currently, notifications only work if a story also has a feed rendering.

Separate "visible in feed" and "visible in notifications", and make notifications query only notifications and vice versa.

Then, set the test notification stories to be visible in notifications only, not feed.

This could be refined a bit (there's no way to have the two views…