R1:c034752578cf

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.