Add helm lint to CI

Related to #115 (closed)

helm lint is a thing and the charts don't currently pass helm's lint checks.

helm lint --set gitlab.certmanager-issuer.email=asdf@asdf.com
==> Linting .
[ERROR] Chart.yaml: directory name (helm.gitlab.io) and chart name (gitlab-chart) must be the same
[INFO] Chart.yaml: icon is recommended

This won't actually lint each of the subcharts. We won't be able to do that until after each chart is fully independent.