Implement 'tag:' style URIs for the GUID field on our RSS feeds. This
ensures new package updates show up as new, and we aren't jumping back
and forth between generated GUIDs having 'http://' and 'https://'
prefixes.
Much of the work here is to attempt to keep old news GUIDs constant so
we don't once again make everything show up as new in newsreaders.
Signed-off-by: Dan McGee <dan@archlinux.org>
