[api] Remove api/v1 directory, move inventory_units controller to correct spot
Revert premature merging of #2414.

Revert "Moved Spree::StoreController class_eval from engine to store_controller_decorator."

This reverts commit 25e3c8521c7eb5a01ac47ca0cd9d5261325d2b43.

Revert "Moved decorators under the app/decorators convention."

This reverts commit 2f5255edbdbb8811c1cca44833389db369153a92.

Revert "Switched decorator loading to the decorators gem."

This reverts commit 689cf55a17e271e73ecd1851a5389d92c2bfc13c.