Use DISTINCT ON/DISTINCT rather than GROUP BY for admin/products query

Conflicts:
	backend/app/controllers/spree/admin/products_controller.rb
	core/app/models/spree/product/scopes.rb

Fixes #2679

Conflicts:
	backend/app/controllers/spree/admin/products_controller.rb
	core/app/models/spree/product/scopes.rb