Add API support for general comments.

This adds support in the API for creating and manipulating general comments.
The API mirrors the commenting APIs for diffs, file attachments, and
screenshots. It includes resources on the review request, review, and reply
levels.

Like other comments, this supports working with text type requests.

Unit tests were also added for all the new resources.

Reviewed at https://reviews.reviewboard.org/r/6431/