Roughly support inline comment suggestions

Summary:
Ref T13513. This still has quite a few rough edges and some significant performance isssues, but appears to mostly work.

Allow reviewers to "Suggest Edit" on an inline comment and provide replacement text for the highlighted source.

Test Plan: Created, edited, reloaded, and submitted inline comments in various states with and without suggestion text.

Maniphest Tasks: T13513

Differential Revision: https://secure.phabricator.com/D21276