Use helm package registry to publish our helm charts

Charts to use this for:

"Our" charts hosted on open.greenhost.net

https://docs.gitlab.com/ee/user/packages/helm_repository/

• dashboard (dashboard!13 (merged))
• local-path-provisioner (local-path-provisioner!3 (merged))
• single-sign-on
• wordpress-helm (wordpress-helm!80 (merged))
• nextcloud-onlyoffice (nextcloud!84 (merged) and nextcloud!97 (merged))

Upstream charts / our forks of them, hosted on github

* [ ] nextcloud-helm, our fork of the upstream nextcloud helm chart. This one is a bit special, because it's supposed to be a temporary fork (because of https://github.com/nextcloud/helm/pull/152), and it's on github, not our gitlab. For both of those reasons we may not want to set up automatic packaging/publishing. Resolved: https://github.com/nextcloud/helm/pull/152 got merged!

* [ ] rocketchat-helm-chart, same story as nextcloud (see https://github.com/RocketChat/helm-charts/pull/21, https://github.com/RocketChat/helm-charts/pull/17)

• wekan (see https://github.com/wekan/wekan/issues/4051 for the progress on publishing an offcial wekan chart repo)
• Zulip

Use helmRepos instead of gitRepos in stackspin flux

• dashboard
• local-path-provisioner
• single-sign-on
• wordpress-helm
• nextcloud-onlyoffice
• wekan
• zulip