Making the width of the phriction preview match the width of the actual content

Summary: Making the width of the phriction preview match the width of the actual content

Test Plan: Edit a phriction article and see that the preview is the correct size

Reviewers: chad, #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T5243

Differential Revision: https://secure.phabricator.com/D9351