Fix admin route rendering

I noticed @plypy has switched admin routes (like /admin/globalnav) to be served by the sub-app, not the core express app configured to render UI.

The trouble with doing it this way is that admin pages don't render the layout properly. And currently, trying to render /admin/globalnav throws an error since the view template expects the `style()` function to be there.

This PR switches admin routes (but *not* user-facing routes like /globalnav) to be served by the core express app.

@plypy, does this look alright to you?