Move shipping_methods and shipping categories from spree_backend to OFN

[luisramos0]

What? Why?

Part of #4050

This moves all things related to shipping methods and shipping categories from spree_backend to OFN.

What should we test?

The code was just moved around so a basic test should do.

Existing bug around User feedback on delete:
The flash message after pressing the delete button is still not showing. It was broken before, I tried fixing it now but didnt manage to. Still not showing.
If ship method is deleted successfully, no flash message to user.
If the ship method is referenced by an order, the ship method is not deleted and no flash message to user is shown. But the ship method will disappear from the list and reappear if user refreshes page 🙈

Release notes

Changelog Category: Changed

Imported views from spree_backend to make OFN more independent of Spree.

[sauloperez]

gogogo!

[daniellemoorhead]

The flash message after pressing the delete button is still not showing. It was broken before, I tried fixing it now but didnt manage to. Still not showing.
If ship method is deleted successfully, no flash message to user.
If the ship method is referenced by an order, the ship method is not deleted and no flash message to user is shown. But the ship method will disappear from the list and reappear if user refreshes page 🙈

@lin-d-hop would this be worthy of an admin UX (or maybe it already is)?

[filipefurtad0]

Hi :-)

What was tested:
Performed general test on Katuma as Administrator, creating user, adding/deleting, managing order cycles.
Created user, ordered a product.

Observed:
All worked fine. Verified the bugs you previously mentioned:

• The flash message after pressing the delete button is still not showing.
• If ship method is deleted successfully, no flash message to user.
• If the ship method is referenced by an order, the ship method is not deleted and no flash message to user is shown. But the ship method will disappear from the list and reappear if user refreshes page

I didn't detect any further errors, from moving the code from spree_backend to OFN.
Moving to Ready to Go! 🎉

[luisramos0]

well done Filipe!