Add comment moderation action endpoints

This includes the flag/unflag endpoints for use by the plot detail page
as well as the moderation endpoints for hiding, archiving, and flag
clearing.

I chose to create an EnhancedThreadedCommentFlag model because the
built-in CommentFlag does not support hiding, and I though having our
doubly-inherited comment model link back to another inherited model
would be too confusing, or perhaps impossible.
Change instance comment base endpoint

I made endpoints for flag and unflag and these look most natural as

POST /{instance}/comments/{comment_id}/flag/
POST /{instance}/comments/{comment_id}/unflag/
Merge pull request #1602 from jwalgran/topic/comment-moderation-backend

Comment moderation backend