Restoration fails when no dashboard is connected

[ian-r-rose]

An unintended consequence of #198. We are being fairly aggressive about disposing of dashboard panels that don't match up with one of the ones in individual-charts.json. But since we removed the default dashboard panels in #198, this means that upon restoration if there is no active cluster, the panels will be removed. Among other things, this prevents the "initial layout" trick we use in dask-examples.

To reproduce:

1. Start a cluster
2. Connect to the cluster using the labextension side panel
3. Open a few dashboard panels
4. Refresh the page. Your panels should be restored.
5. Shut down the cluster. The panels will be deactivated, but won't go away.
6. Refresh the page. The panels will not be restored.