Currently, we have logic in `ExtensionManagerView` that shows a loading spinner until that view's model is finished initializing. That no longer shows up because (1) we don't show either view until all the models are done loading and (2) even if we did show the first view (installed items), that one will probably complete before the registry view's model is done loading, so we wouldn't see its spinner anyway. So I think we should hoist the spinner display up into the dialog, since it's the dialog's `modelPromise` that we want to wait for to shut off the spinner anyway.