Prepare for updated Resolver.

* Makes `config/environments.js` into an ERB file.
* Uses `copy_file` instead of `template` in `ember:boostrap`. `template`
  is used to evaluate the template at generator runtime (which is NOT
  what we want).
* Centralizes the namespace configuration into
  `config/environment.js.erb`.
* Adds a custom method to the Resolver for `resolveSerializer`.
* Uses `typePrefix` to resolve adapters, serializers, and the Router.

There is a PR pending on the resolver repo to make the behavior in
`resolver_ext.js` not needed at all (we can remove it).