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. Testing Done: 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.