This commits adds support for metrics dashboards
for embedding. If the flag 'embedded' is provided
to the environments/id/metrics_dashboard endpoint,
the response will be suitable for embedding in
issues or other content.

This is a precursor for support for embedding
metrics in GFM.

Adds a new stage to dashboard processesing step for the
EnvironmentsController::metrics_dashboard endpoint.

Allows the front end to avoid generating the endpoint
unitutive string mutations.

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.

Either the query or query_range attribute needs to be present in the
metrics yml. The yml is invalid if both are present or neither is
present.

Validate the schema of the dashboard that is loaded by
Gitlab::MetricsDashboard::Service.

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