Revert "Remove append_view_path from api ControllerSetup"

This reverts commit 92aaf03a787fc37810c07a1f8605fc4f1bb73c29.
Bump to allow any highline version in the > 1.6.18 & < 1.7.x range

Fixes #3870
Fix issue where Product#set_property was causing an undefined method with spree_i18n

Fixes spree/spree_i18n#301

Conflicts:
	core/app/models/spree/product.rb
Select2 fixes to center the search box

Fixes #3854
Fixes #3863
Add process method to BogusSimple gateway to be able to process payments.

Fixes issue where hitting "process" on payments screen breaks with
BogusSimple gateway.

Fixes #3824
Fix issue where Product#set_property would sometimes complain about a missing property association
checkout/registration view should live in auth component, not frontend
Bump activemerchant to 1.40.0

Likely fix for spree/spree_gateway#105
Fix sporadically failing new_order_spec
Split creating payments + void test into two separate tests
use correct casing in payments_spec for 'paid' check
Add index to Spree::User.spree_api_key

Fixes #3884
Updated docstring explaining how to include controller requests test support in rspec.

Fixes #3898
Fixed Spree::Api::ProductsController#index to accept multiple ids

Fixes #3886
Relax money dependency
Bugfix: Taxons could not be ordered

If you reorder the taxons in the admin interface, it does not
have any effect, because the ordering of the taxons is not
used. Setting a default scope with this ordering fixes that.

Fixes #3891
Fixes #3833
Fixes #3390
Spree::Payment.class.undescore will always convert spree/payment

Fixes #3889

Conflicts:
	backend/app/views/spree/admin/payments/show.html.erb
Fix fieldset not resizing on some @media sizes.

Fixes #3871
Fixes #3890
[api] Don't depend on ordering in taxons_controller_spec
[api] Add missing per_page node to products/index