Save drafts for inline comments currently being edited

Summary:
Ref T13513. As users type text into inline comments, save the comment state as a draft on the server.

This has some rough edges, particularly around previews, but mostly works. See T13513 for notes.

Test Plan: Started an inline, typed some text, waited a second, reloaded the page, saw an editing inline with the saved text.

Maniphest Tasks: T13513

Differential Revision: https://secure.phabricator.com/D21216