Add preview and drafts to audits

Summary: Add comment previews and saved drafts to audits, like Maniphest /
Differential.

Test Plan: Typed stuff into the box. Got a preview. Reloaded page. Stuff was
still there. Submitted comment. Stuff is gone.

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T904

Differential Revision: https://secure.phabricator.com/D1699