Tweak the visuals for DnD overlays a bit.

Review Request #8174 — Created May 20, 2016 and submitted — Latest diff uploaded


Review Board


The original implementation had the overlays just change the border color when
hovering over them to drop. This wasn't a particularly noticible transition,
especially given the other colors we use on the page. This change switches that
over to just lightening the background color instead.

While I was doing this, I also fixed the code to cope with padding, which was
an issue on the file attachments overlay when there were existing file
attachment thumbnails present.

Did some drops.