mgr/dashboard: The "Create" button on the RBD page should be disabled if no rbd Pool exists
If no Ceph pool with the "rbd" application label has been created, the dashboard should not make it possible to open the "Create RBD" form, as the user won't be able to proceed with it anyway (because there is no pool to select).
Instead, the "Create" button should be greyed out and a tooltip should indicate that no Pool with the "rbd" application label exists (e.g. "Unable to create image. No pool with the "rbd" application label available.").
This is less frustrating than starting a workflow and being stopped somewhere mid-flight with something that should have been obvious before.