Publish Helm Chart in CircleCI

[Gaardsholt]

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Title: Update release process and chart packaging

High level description: This pull request updates the release process and chart packaging.

Summary:

• The release process is now triggered only on "chart-*" tags.
• The chart packaging process now uses cardinalby/git-get-release-action@v1 to get the latest chart and app releases.
• The chart packaging process now replaces strings in charts/nightscaler/Chart.yaml with the latest app and chart versions.
• The chart packaging process now installs chart-releaser v1.6.0.
• The chart packaging process now packages the chart and uploads it to the release.
• The chart packaging process now updates the chart index.
• The release-drafter/release-drafter action is now used with the release-drafter-helm.yml config file.
• The GITHUB_TOKEN secret is now used to authenticate with GitHub.
• The NIGHTSCALER_TOPIC environment variable has been removed.
• The NIGHTSCALER_CLUSTERNAME environment variable has been removed.
• The NIGHTSCALER_LISTEN_TOPIC and NIGHTSCALER_PUBLISH_TOPIC environment variables have been added.
• The NIGHTSCALER_CLUSTERNAME environment variable is now optional and can be set using the .Values.pubsub.clustername field.
• The topic field was renamed to listen_topic.
• A new field publish_topic was added.