"Start new discussion" button on view-discussion page

[ewinslow]

It occurs so often that a certain thread of discussion sparks a related-but-different idea in folks minds. This is great and we want to encourage this kind of discussion-spawning-discussion activity, but we don't want distinct conversations getting buried within one another. To encourage people to keep things clean, it seems like it would be best if we had a "Start a new discussion" button there that linked back to the original for context.