Until now, my plan was to get as much of the status update code in as possible

but never move the future out of EXPERIMENTAL until we had the new unified

draft banners. This was because the reply banners for replies to reviews were

owned by the ReviewBoxView, and there seemed to be no good way to port that

behavior over given that there are multiple reviews stuffed into a single box.
This change makes it work by displaying the reply draft banners inline in the

status update sections, below the header (which contains the status update

summary) and above all the comments. This is set to float so that it will dock

at the top of the screen as long as that review is visible, the way that our

other reply draft banners work.
The goal is still to replace these with the new unified draft banners, but that

work is no longer blocking.
While I was in here I fixed a small regression that I introduced with

collapsing review boxes that have a draft reply (where the banner would still

be displayed).

Replied to reviews attached to status updates, both in the initial status

  updates box and the change description boxes.
Verified the floating banner behavior when scrolling around with a bunch of

  reply drafts.
Checked the appearance of all types of boxes when expanding and collapsing.