templates/html/show.tmpl: Make "Version" column of download table more readable

Previously it was sometimes difficult to spot differences in the versions,
especially if the are only in the Debian revision. Color code the
background so that one can spot differences easier. All up-to-date
versions are indicated with a green background. If the version differs
from the latest version only in the Debian revision, use yellow.
If they differ in the upstream revision, use red.

Inspired by discussion with Neil Williams <codehelp debian org>