Since it's no longer tied to the "Reply" button, we can move the code
from ReplyWidget to NewTopicController, where it belongs.
Bug: T272543
Change-Id: I8c90ffb772573d22d26c608d45877ee948fd232d
1. Extend the JS modifier to allow adding top-level comments
(that is, replies to headings). PHP modifier doesn't do this
because we'll save the changes using paction=addtopic instead.
2. Subclass CommentController to allow adding a new heading and a
top-level comment underneath it at the same time.
3. A lot of ugly code in ReplyWidget to customize the interface
for this case. Much of it should probably be moved to
CommentController/NewTopicController.
Bug: T267595
Change-Id: I9c707bb7f7aae1b92c72fb4dee436490f8c8409b
