Pkgdb: Avoid redirects to URL with trailing slash

/pkgdb/package/foo redirects to /pkgdb/package/foo/, therefore use the
final URL with trailing slash in links.
Merge pull request #92 from tyll/avoid-redirects

Pkgdb: Avoid redirects to URL with trailing slash
Pkgdb: Make package retirement messages explicit

This addresses:
https://github.com/fedora-infra/fedmsg_meta_fedora_infrastructure/issues/90