Prevent archive zombies

Interrupted archive downloads were creating zombies, see
https://gitlab.com/gitlab-org/gitlab-workhorse/issues/9

See merge request !25

Episode V: The Archive Zombies Strike Back

The -relativeURL option existed for a while but was never included in
an official release.

Do not show error pages in development mode



See merge request !24

Fix routing for encoded slashes

It seems URI.Path gets unescaped. URI.EscapePath() contains the `%2F` we want.

Also see https://gitlab.com/gitlab-org/gitlab-ce/issues/4124

See merge request !23