Add a "feed" filter to the home page; align things; allow browsing older stories

Summary:
Pretty straightforward; see title. Kind of gross but I have a bunch
more iterations in mind here (like filtering). Paging this is a little tricky
since we can't easily use AphrontPagerView, as it relies on OFFSET, and I think
that's sort of sketchy to use here for UX reasons (query performance and view
consistency as feed updates).

Test Plan: Looked at feed, paged through feed.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1616