gitlab: 8.0.5 -> 8.5.0, service improvements

Updates gitlab to the current stable version and fixes a lot of features that
were broken, at least with the current version and our configuration.

Quite a lot of sweat and tears has gone into testing nearly all features and
reading/patching the Gitlab source as we're about to deploy gitlab for our
whole company.

Things to note:

 * The gitlab config is now written as a nix attribute set and will be
   converted to JSON. Gitlab uses YAML but JSON is a subset of YAML.
   The `extraConfig` opition is also an attribute set that will be merged
   with the default config. This way *all* Gitlab options are supported.

 * Some paths like uploads and configs are hardcoded in rails  (at least
   after my study of the Gitlab source). This is why they are linked from
   the Gitlab root to /run/gitlab and then linked to the  configurable
   `statePath`.

 * Backup & restore should work out of the box from another Gitlab instance.

 * gitlab-git-http-server has been replaced by gitlab-workhorse upstream.
   Push & pull over HTTPS works perfectly. Communication to gitlab is done
   over unix sockets. An HTTP server is required to proxy requests to
   gitlab-workhorse over another unix socket at
   `/run/gitlab/gitlab-workhorse.socket`.

 * The user & group running gitlab are now configurable. These can even be
   changed for live instances.

 * The initial email address & password of the root user can be configured.

Fixes #8598.

nixos/modules/rename.nix

@@ -28,6 +28,9 @@ with lib;
     (mkRenamedOptionModule [ "services" "subsonic" "host" ] [ "services" "subsonic" "listenAddress" ])
     (mkRenamedOptionModule [ "jobs" ] [ "systemd" "services" ])
 
+    (mkRenamedOptionModule [ "services" "gitlab" "stateDir" ] [ "services" "gitlab" "statePath" ])
+    (mkRemovedOptionModule [ "services" "gitlab" "satelliteDir" ])
+
     # Old Grub-related options.
     (mkRenamedOptionModule [ "boot" "initrd" "extraKernelModules" ] [ "boot" "initrd" "kernelModules" ])
     (mkRenamedOptionModule [ "boot" "extraKernelParams" ] [ "boot" "kernelParams" ])