Build GeneralComment Model

Review Request #6375 — Created Sept. 26, 2014 and submitted — Latest diff uploaded

Review Board
reviewboard, students

Reviewboard currently support 3 types of comments: Diff commment, File attachment comments, Screenshot comments.
This is a new type of comments: General comments.

A general comment on a review request is used when a comment is not tied to specific lines of code or a special file attachment, and an issue is opened. Examples include suggestions for testing or pointing out errors in the change description.

This review request is for GeneralComment backend model.

Five test added in reviews/
1. test_review_detail_general_comment_ordering
2. test_init_with_general_comments
3. test_init_with_mix
4. test_init_general_comment_with_replies
5. test_save_reply_comment_to_general_comment

All test passed.

Previously test 1 failed due to use diff viewer 'get' method on general comments.
Figured out general comments behave the same as screenshot/file attachment comments.