Set a max-height on codeblocks in Conpherence

Summary: Code blocks in Conpherence can be massive and disruptive to a chat-like experience. This caps the height.

Test Plan: Add a ginormous code block to Conpherence, see it hit max-height

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D13790