Extracts the JSON response from the admin's line item controller which
are only used by the bulk management feature into its own controller.
This decouples spree from an OFN-only feature and allows to remove
unnecessary code. Furthermore, Admin::LineItemsController is gone in
Spree 2.4.0.beta. See: https://github.com/spree/spree/pull/5280
@order is not loaded by Spree because we overrode the :before_filter in
spree/admin/orders_controller_decorator.rb. We need to also specify so in the
decorator.
Spree does not call after_<order.state> methods any more as of
https://github.com/spree/spree/pull/2557, so our #after_complete method
is never triggered and thus the order never reset.
This makes the condition:
```ruby
if current_order.andand.distributor == @order.distributor
```
in app/views/spree/orders/form/_update_buttons.html.haml return false
and as a result the "Back To Cart" button is not shown.
This commit resets the order (emptying the session[:order_id] and
creating a new order, aka. cart) right from the
CheckoutController#update rather than relying on infernal callbacks (of
what the Spree core team itself was unhappy about since long ago
https://github.com/spree/spree/issues/2488). There is the first place
where we know the order has been successfully completed.
In the step 6 of the spree upgrade, when deleting an order's line item
from /admin we get an HTML response (the whole edit form) that gets
inserted into the flash error.
It's not 100% clear why we added a respond_to block for JSON but since
juery-rails sends ajax requests with application/javascript, */* in
the ACCEPTS request header, Rails falls back to the first specified respond_to
block, which is HTML. We don't hit the JSON block.
In order for this to work, this commit replaces the JSON respond_to
block with one for JS with the exact same behaviour; returning a 204.
This is also the format the spree controller action we duplicated originally
has besides HTML.
