Add links to enable downloading of tagged blobs

All tags below refs/archives are shown on the repo summary page as
download links. The links referes to the tagged objects, using the
tag name as filename for download.

This can be used to add shortcuts for release tarballs, documentation
and other blobs stored in the object database, especially blobs that
are not reachable during cloning.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>