Fix regressions caused by the recent editor changes.

Some of my recent work made assumptions about elements that exist on a
page in ReviewRequestEditorView. While fixing this, I realized there's a
lot of work we're doing on those pages we don't need to do.

This change conditionalizes a lot of the logic to only work with fields
and other review request editable stuff if we're on a page with those
elements. We could get more fine-grained, but this is enough to simplify
review UIs a fair amount and to fix the breakages.

It also fixes loading images in a Markdown field. Since an image could
load after we render (unless it's in cache), it could end up messing
with the sizes. Now we call resizeLayout when any contained image loads.

There's also fixes to make unit tests pass. The structure of the
template better matches the real structure of the review request, at
least close enough for logic to work.