Updates the EnvironmentController#metrics_dashboard endpoint
to support a "dashboard" param, which can be used to specify
the filepath of a dashboard configuration from a project
repository. Dashboard configurations are expected to be
stored in .gitlab/dashboards/.

Updates dashboard post-processing steps to exclude custom
metrics, which should only display on the system dashboard.

- Order groups by descending order of priority.
- Order panels by descending order of weight.
- Perform sorting after adding project/custom metrics.