Per @lunny's feedback, rework to reuse the existing Comment table
instead of a standalone commit_comment table. The junction table
(commit_comment) now only stores repo_id, commit_sha, comment_id.
Actual comment data (content, tree_path, line, patch, poster) lives
in the Comment table with Type = CommentTypeCommitComment (39).
This gives commit comments reactions, attachments, and all existing
comment infrastructure for free.
