Shell chart
This MR replaces https://gitlab.com/charts/helm.gitlab.io/merge_requests/32 due to the fact that we can're reuse release names, and causing the review environment to fail.
Closes: https://gitlab.com/charts/helm.gitlab.io/issues/55
Adds the gitlab-shell chart.
Requires an additional manually configured secret to be installed into the cluster ahead of time. I've added the steps for this to the docs for now.
Edited by DJ Mountney