DiffViewer: View deleted images

DiffViewer: If the image was deleted, load the image from the
 base revision; show file status of the current image with stripes
 down both sides.
themes: Add missing card background for dark theme
Let ZipImageRequest calculate its own URL
Added StripedImageView to support showing the file status with
 an image
PatchSetChangesCard: Enable card when tags are set

Patch Set 2: Minor layout change to better handle small images, see
 comments for before|after

Change-Id: I991fac3d53168c39af652a335a7e4ef0772495be