Global dashpages

We want to make it easier for the dashboard administrator to expose highly requested data to their users.

For example, ACA users have requested the ability to see Envoy metrics.

As part of the dashpage configuration model, we will allow specifying a resource name on a chart. The chart will then always show this resource, if the resource and metric are available. A "global" dashpage is one for which all charts specify a resource name.

Today, the first click on the "Metrics" tab in a user's session produces a largely empty screen. An extra series of actions are required to select a resource in order to see anything:

With the introduction of dashpages, we have the opportunity to show metrics whenever the required resources are available.

Engineering work for this:

- Add metadata to a dashpage chart for an optional (nullable) resource name.
- Per #5295, we will only show dashpages. Adding global dashpages will require extending those checks to test whether all specified resources are available.
- Update the UI to show navigation items when no resource is selected, but a global dashpage is available.
- Showing a message to the user that they can select a resource to show more options. This could appear below the navigation tree in the current UI.
- Differentiate global dashpages in the navigation tree?
- Add a group at the top of the navigation tree.
- Use an icon alongside their name such as 🌐.