Clean up the look of review UI pages.
Review UI pages were pretty ugly. They had this bit of the review

request box sticking out with nothing but the caption. This was from

back when we filled that with the screenshot list.
Now, by default, a review UI using the default.html template or the

review_ui_content_box block will have its UI shown in the confines of

that box. It has the whole area to play with, and isn't restricted by

any padding or UI elements. It will automatically have the actions

drop-down right above it as well.
Review UIs not using this template or block will appear below as before.

In this case, though, the bit of box will be removed, showing just the

action bar, cleanly floating there.
One side-effect of this is that the caption is no longer shown

automatically. It's now up to the review UI to decide how to show this.

Tested review UIs that made use of the default template and could go in the box (image and a custom one I'm building), and ones that didn't (PDF).
Tested that image diffing was unaffected.
Tested screenshots.