Add a document preview to Phriction

Summary: Preview Phriction documents as they are edited, similar to how
Differential/Maniphest work.
Test Plan: Mashed my keyboard while editing a Phriction document.
Reviewed By: hsb
Reviewers: hsb, codeblock, jungejason, tuomaspelkonen, aran
CC: aran, hsb
Differential Revision: 684