Remove unnecessary frontend/backend query scoping on ShippingRate

Related to #4067
Only sort available shipping rates in Spree::Stock::Estimator#shipping_rates

Related to #4067
scheme-less URL generation fix

If the config setting is blank, we need to pass paperclip an empty string not nil

Fixes #4060
Fixes #4068
[sample] Correctly assign products to brands in taxons

Fixes #4070
Only choose default shipping rate if provided shipping rates aren't empty
Add test to ensure that backend rates are not returned in Spree::Stock::Estimator#shipping_rates

Also fixed up the other test in this block.

Related to #4067