Check if the intents_controller should live in the extension or among templates

[elia]

Why?

The controller is really simple and using well defined extension APIs. Everything else might be heavily dependent on the specific store setup (e.g. single page checkout, Ajax endpoints, etc.). So we might want to make it easy to adapt it to the store's needs.

What

Move it from app/controllers to lib/generator/…/templates.

[elia]

Deferring to the next minor.