[Redesign] Set a max-width on Graphviz

Summary: Ref T8750, Adds a maxwidth class for Graphviz images.

Test Plan: Generate a Graphviz image, really big, see it scale to the viewport.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T8750

Differential Revision: https://secure.phabricator.com/D13548