Make feed its own application

Summary:
Fixes the two-level nav issue introduced by D4376.

(My claim that this page is device ready in the code is something of a lie, but it's fairly close.)

(@chad, this could use an icon at some point, or you can point me at which one you want and I can take a stab at slicing it.)

Test Plan: Looked at feed; saw it not-broken. Also checked public feed (which should just merge at some point).

Reviewers: btrahan, chad

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D4381