The image review UI has something of a UI quirk about where we stick the

resolution drop-down menu. For attachments, we either stick it in the

diff mode bar (when viewing a diff), or the caption (when viewing a

single revision). This didn't work when the review UI was embedded

inline in the diff viewer. Actual diffs for images worked fine, because

we had the diff mode bar, but newly-added image files didn't (since

there's no mode bar and no separate caption bar). This change adds an

empty mode bar to use when we're in that situation.

Viewed a diff that contained a newly-added image file. Saw that I had a

mostly-empty toolbar that allowed me to select the zoom for the image.