App is not a singleton but app of scaffolded app should be the only root instance

[sio4]

As reported at gobuffalo/buffalo#1653, in very rare situations, App() can be called multiple times while app is still nil. However, App itself is not a singleton from the perspective of the library (Multihoming or Group uses the same App), it should be prevented by the user's application. (gobuffalo/buffalo#1653 (comment))

The scaffold command buffalo new generates without this prevention, so the template should be updated.

fixes gobuffalo/buffalo#1653