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

Hat tip to Stan Hu:
https://gitlab.com/gitlab-org/gitlab-workhorse/merge_requests/21

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