Use `unscoped` when create stock items in migration

By the time the migration runs the default scope with deleted_at was
already defined so mocking acts_as_paranoid doesn't make any difference

related to #3805
Revert "Bump aws-sdk to 1.20.0"

This reverts commit c0471ef33faeaa46e1fec943e4cf3526437ee16d.

This version unfairly locks nokogiri to 1.6.0, which may cause issues with Deface.
Ensure that users cannot set price on line items through the API
Fix format of price input in products/new.html.erb
[frontend] Stop people encountering uninitialized Taxon errors when they override ProductsController#show
Remove regression tests for #3404

These were proving extremely difficult to test, and so I have removed them. This issue should no longer come up, anyway.
[api] Remove 'add line items' test from orders controller

if people want to add line items, they should do it through the line items endpoint
Non-GET requests that would usually non-ssl-redirect will now respond with HTTP status 426
Include exception message from Spree::Core::GatewayErrors

This makes the error messages when payments fail slightly less vague than they used to be.

Fixes #3851
Bump activemerchant to 1.39.2

Conflicts:
	core/spree_core.gemspec
[backend] Use the controller name rather than Object when a model does not exist for authorize_admin method

Fixes #3622
Move preference_resuce.rb into Core, where it should've been

Fixes #3860
Fix issue where cancelling a line item delete didn't actually cancel it

Fixes #3862

Conflicts:
	backend/app/views/spree/admin/orders/_shipment_manifest.html.erb
Do-over "Add Stock Movement" form

If there are a large number of stock items for this stock location, the page can take some seconds to load, as mentioned in this thread:

https://groups.google.com/forum/#!topic/spree-user/nu0_NlNGPV8

To make that stop being a problem, I've switched that UI to using select2 to load stock items 50 at a time. I've also imporved the UI to show more information about the product, such as its SKU and its options text.
[backend] Include options text when displaying variants on the stock movements page
[api] Add eager loading for option values + option types to stock items

Conflicts:
	api/app/controllers/spree/api/stock_items_controller.rb
[api] include option_type presentation within option_value_attributes
[backend] More clearly differentiate select2 result elements
[backend] Split variants/_autocomplete into two templates which can be used independently
Bump to 2.0.6