Add validation for #3494
Create a custom router mapper so we have more control over the routes

Due to reasons explained in rails/rails#12367

Fixes problems like spree/spree#3757
Revert "Create a custom router mapper so we have more control over the routes"

This reverts commit b8d7e9ba27ae00aa3bdd49289bcddd6a2bb355b1.
[api] Ensure that users cannot set prices on line items during creating an order or updating it