[api] Support order routes on /api/checkouts as well as /api/orders

I got sick of not knowing if /api/checkouts was the right prefix to use, or if I should use /api/orders. If I get sick of it after one day of usage, then I'd suspect other people who are using the API would also be sick of it.

Let's fix this problem and there will be no more getting sick of it :)
[api] Allow for accessing states inside of countries in the API

Strangely, StatesController already supported this but there wasn't a route defined for that. I only found this out today when I tried making a request to /api/countries/:country_id/states (with my spree_api_examples project) and had that fail.
[order importer] Assign created shipping rate as the selected shipping rate
[order importer] Ensure that order totals are correctly updated
Mulder
Allow for assigning a user_id to an order

This now means that you can create a user through the API and then link that user to an order
Add changelog for 35fa88b589ef4f935dc9045f48a782d89b56da11
[backend] Make variant selection in _create_line_items wider
Include display_tax_total in API responses
Touch an order when a line item is saved

Fixes #4419

Conflicts:
	core/app/models/spree/line_item.rb
	core/spec/models/spree/line_item_spec.rb
backend: Remove missing reports#show route

Fixes #4528
More robust namespace adjusting

As discussed in https://github.com/spree/spree/issues/4479, the existing migration would generate inaccurate class names due to the fact that `demodularize` will only select the very last part of the class name.

So, for instance,

`Spree::Calculator::Fedex::Ground` would be changed to
`Spree::Calculator::Shipping::Ground`, which would trigger an `uninitialized constant` exception. The desired new class name is
`Spree::Calculator::Shipping::Fedex::Ground`, and this suggested PR will handle those class name changes more appropriately.

Fixes #4520
Adding data-hooks to admin shipping method form

Our app does some rearranging of these elements, and it's dramatically easier if not only the group of fields, but the fields themselves have `data-hook` defined. This should help anyone working with this group of fields in Deface.

Fixes #4521
Set order_id parameter explicitly in checkouts_controller and orders_controller to params[:id]
Check for has_spree_role? using a string 'admin'

This makes a test in api in orders_controller_spec happy
[backend] remove csrf-meta from partial bodies

Fixes #4531