Global dashboard - Use JupyterHub's active users metric instead of counting pods

[consideRatio]

We have current / daily / weekly / monthly users in the JupyterHub Dashboard, under the "Hub usage" row:

They rely on JupyterHub's metric doing the counting of users.

In the global dashboard though we count pods instead I think, but that ought to be more complicated because you need to check for all pods popping up and going away etc.