createComment(dumpId: string,userId: string,body: string,parentId?: string): Comment
Posts a comment on dump dumpId, replying to parentId if given, for
POST /api/dumps/:dumpId/comments. It notifies the dump's poster and the
users it mentions, unless the dump is private, and links the comment's
attachments and backlinks. The route has checked that the caller may see
the dump.