Refactor API user loading out to own method.

This simplifies the logic around what the current user is.

There is now an assumption that the current logged in user is user to
use permissions for.  The secondary user is the user whose API key is
specified.

If both an API key are specified, and a user is logged in, the logged in
user takes precedence.

Fixes #4440
Remove authentication check.

This authentication check is performed in a before_filter only when
requires_authentication is true, so it should not be false here.

Fixes #4440
Remove order_token method.

It is specified in base_controller and is being overridden
unnecessarily.

Fixes #4440
Test order authentication.

This code was previously untested.

Fixes #4440
Authorize an order through cancan.

Fixes #4440
Integrate API key checking into authenticate_user

It doesn't really do much currently.

Fixes #4440
Fix case where 'Free Shipping' was incorrectly displaying

Fixes #4306
API BaseController inherits ActionController::Base

Spree::Api::BaseController now inherits from ActionController::Base.

Removed unnecessary includes

Fixes #4418
Ensure correct Responder / responses are being sent out in the API

Now that we've switched to using ActionController::Base for Spree's API controllers, there was an issue where Spree's API responder was being an ignored, which is now fixed with the changes to respond_with

Unfortunately this broke the custom template stuff, which there were tests for within zones_controller_spec. This functionality is fixed by the override of api_behavior within the RablTemplate module. This method was calling out to ActionController::Renderers and was rendering stuff using code like resource.to_json, which is terrible. What we wanted it to do is render the proper template, and it was only by accident that I found out that doing nothing in that method causes it render the damn default template.

Happy accidents are fun.
Remove only_one_promotion_per_product validation from product promotion rule
Check for EITHER current_spree_user or spree_current_user in try_spree_current_user

I had some requests to the API from the backend which were 401'ing mysteriously... until I worked out that Devise's method is called current_spree_user. I didn't want to break backwards compatibility, so now this method just checks for either spelling of the method.
Stub out correct call in users_controller_spec to set the current user

Conflicts:
	api/spec/controllers/spree/api/users_controller_spec.rb
Implicitly set responder for Spree::BaseController

This ensures that Spree::Responder (rather than ActionController::Responder) is used