Move gitlab-shell to make Go development easier

For Go development we want to have a GOPATH directory layout for gitlab-shell.

Because this is confusing and distracting for Ruby developers, this change puts the GOPATH at /go-gitlab-shell, and makes /gitlab-shell a symlink to the real directory where the gitlab-shell repository is.

Discussion about Go in gitlab-shell: https://gitlab.com/gitlab-org/gitlab-shell/issues/80

Gitlab-shell Go merge request: https://gitlab.com/gitlab-org/gitlab-shell/merge_requests/127