R1:e14ba5639479

Add top border back to section-headers

Summary: This adds back the top border on section headers and cleans up the tab CSS just a hair.

Test Plan: tested files, tasks, and custom field profile.

Reviewers: epriestley, btrahan

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7378