resolveChannelReply applied discussion-thread logic (reply_to_top_id =
the replied-to message's own id) to forum replies. Replying to a General
message produced reply_to_top_id = <that id>, a topic no client can
resolve: the reply vanished from every topic view and reply-jump on
strict clients said "message doesn't exist".
Forum replies now inherit the target's topic via domain.ForumReplyTopicID
(target's topic, or its own id if it's a topic-create, else General), and
General (topic 1) is accepted as a valid virtual topic everywhere, so
sends carrying top_msg_id: 1 are no longer rejected. Non-forum discussion
threads are unchanged.
