Change mix template to register the new application's supervisor.

Why?

a) because that way if you by accident start the supervisor a second
   time it fails early instead of crazy stuff happening when it starts
   its children a second time,
b) because then you get the name of the process in the error logs and
   in observer and everywhere else which is awesome and
c) because it’s convention to register the app’s supervisor under its
   module name.