Reply adjustments, especially for tangents.

* Always make the reply form appear after replies instead of before.
  This makes sense because that is where the new reply would appear.
* Re-exposes the "reply" button for tangents, but the button instead
  opens the reply form for the tangent post's parent post, and hides
  the "topic-level" reply form in order to reduce the user confusion
* Scrolls the viewport to the reply form so users know where to type

Change-Id: Iaa6686bb846d1cd4ad256a8ba4a385fe53eb7899