implementes creating retailer pricing report for product groups in
addition to individual products
  * adds method to product decorator to return product costs
  * wires up generating the report for product groups
pt-78388022 updates to allow admin to generate report of retailer pricing by SKU
Merge branch 'list-all-retailer-pricing-for-given-sku' into staging

Conflicts:
	app/jobs/report_creation_job.rb
	app/models/spree/product_decorator.rb