Comment resources are currently child resources of reviews. This limits the type

of queries we can do for comments, for example we can't query for all diff

comments created by a given user.
This change adds top-level API endpoints for comments. We register root list

resources for diff comments, file attachment comments and general comments and

support GET requests for the resources with the following query parameters:

- review-request-id=<review-request-id>, review-id=<review-id>,

user=<user-username>, is-reply for all

- file-diff-id, line and interdiff-revision for diff comments

- file-attachment-id and file-name for file attachment comments
This change also improves the docstrings and code readability for base diff

comments, base file attachment comments, and base general review comments.
Based on work by Chaoyu Xiang at /r/12037.

Created GET method unit tests for the new resources and ran them successfully
Manually tested the /api/general-comments/, /api/diff-comments/ and

api/file-attachment-comments endpoints