Support comments as an EditEngine field

Summary:
Ref T9132. This adds an automatic "Comments" field, like the Subscribers/Projects/Policy fields.

The primary goals here are:

• Allow users to make comments via Conduit.
• In the future, get stackable action support.

As a side effect, this also allows you to put comments on create forms. This is a little silly but seems fine, and may be relevant on edit forms (which I'm not 100% sure how I want to handle yet). I've just hidden them by default for now.

Test Plan:
{F976036}

{F976037}

{F976038}

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9132

Differential Revision: https://secure.phabricator.com/D14515