Add a "Pro" version of the Differential comment save controller

Summary: Ref T2222. Adds a mostly-functional "Pro" comment controller. This does the core stuff, but does not yet do actions (accept, reject, etc.) or inline comments.

Test Plan: Changed the `if (false)` to an `if (true)`, then made some comments, etc. This is normally unreachable.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8304