Bring return authorizations controller from spree_backend

app/controllers/spree/admin/return_authorizations_controller.rb

@@ -0,0 +1,24 @@
+module Spree
+  module Admin
+    class ReturnAuthorizationsController < ResourceController
+      belongs_to 'spree/order', find_by: :number
+
+      update.after :associate_inventory_units
+      create.after :associate_inventory_units
+
+      def fire
+        @return_authorization.public_send("#{params[:e]}!")
+        flash[:success] = Spree.t(:return_authorization_updated)
+        redirect_to :back
+      end
+
+      protected
+
+      def associate_inventory_units
+        (params[:return_quantity] || []).each do |variant_id, qty|
+          @return_authorization.add_variant(variant_id.to_i, qty.to_i)
+        end
+      end
+    end
+  end
+end