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 render different values today, for example) but since the only actual use case we have right now is test notifications I don't want to go too crazy future-proofing it. I could imagine doing some more of this kind of stuff in Conpherence eventually, though, perhaps.

Test Plan: Sent myself test notifications, saw them appear on my profile timeline and in the JS popup, and in my notifications menu, but not in feed.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13222, T10743

Differential Revision: https://secure.phabricator.com/D19864