Minor visual tweaks to feed.

Summary: Normalizes the font-size for readability, consistent padding to the header, more prominence on the date dividers.

Test Plan: Reviewed in sandbox. http://phab1.pushlabs.net/feed/

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4402