Make code blocks clear floats in Phriction

Summary: In situations where a code block and a ToC are displayed, this clears the float so the code block is always 100% wide.

Test Plan: Tested a code block with a ToC, iOS and Chrome

Reviewers: epriestley, btrahan

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D6004