Move ToC in Phriction to it's own column

Summary: Adds additional CSS to make the Table of Contents into it's own column (if one exists).

Test Plan:
Tested a page with and without a table of contents. Tested tablet, mobile, and desktop breakpoints. Tested Conpherence, scrolling seemed fine still on trackpad and mouse.

{F370973}

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12417