dashboard chart: split into application and runtime parts

[petersutter]

What would you like to be added: The dashboard chart in its current form does not conveniently support a "virtual garden" setup and assumes a single cluster deployment.

Why is this needed: The chart should be split into an application chart (for the virtual garden) and a runtime chart for the runtime cluster. We can then adapt the gardener-dashboard-role accordingly https://github.com/gardener/dashboard/issues/545. Currently, it's not possible to deploy the gardener-dashboard-role using the chart to the virtual garden.

[petersutter]

/assign