Use a simpler footer style to accommodate overly wide pages

Summary: Not really thrilled about my fix for T684 in D1224. This makes some
design tweaks to solve it without the awkward horizontal scrollbar in the page
content div.

Test Plan: Looked at diffs overflowing the window. Looked at footer on several
pages.

Reviewers: btrahan, jungejason, Makinde

Reviewed By: btrahan

CC: aran, btrahan

Maniphest Tasks: T684

Differential Revision: https://secure.phabricator.com/D1332