use .try on @differentiator into delivery step

in order to prevent NoMethodError when it is nil

Fixes #3984
Make api helper attributes class attributes

To make it easier to append more attributes. Other applications /
extensions can append their own stuff without having to override
methods. e.g.

        Spree::Api::ApiHelpers.order_attributes.push :locked_at
Make api helper attributes class attributes

To make it easier to append more attributes. Other applications /
extensions can append their own stuff without having to override
methods. e.g.

        Spree::Api::ApiHelpers.order_attributes.push :locked_at
Use templates for option_values + option_types in products/show to DRY up things
[api] Cache products/show view

This can reduce the time it takes for a request to /products from > 1 second to < 75ms

Before: 1048.7ms
After: 65ms

If a product property, option value, option type, image or variant of a product changes then the product will be touched which will invalidate the cache.

Conflicts:
	core/spec/models/spree/variant_spec.rb
Clear CHANGELOGS for 2.1.4
Added changelog entry for cache improvements
Set postgres host when creating a test_app

Fixes #3988
Remote erb inline comment from backend/payment_method/form

Motivation is to avoid parse errors when trying to deface that file

See #3913
Allow inventory tracking control (enable/disable) at the variant level. This retains the site-wide inventory tracking flag, but also allows this type of control down at the variant level.

Fixes #3974

Conflicts:
	core/app/models/spree/stock_item.rb
	core/app/models/spree/variant.rb
	core/spec/models/spree/variant_spec.rb
Use different id on hidden field so associated labels work as expected

Fixes #3981
position should not be present in taxon_attributes
Taxon sorting for 2-1-stable fix

Fixes #3989
use a better name for the method that restarts the checkout flow,also make sure we use the checkout_steps.first instead of hardcoding it to "address".

use expect syntax

use the correct syntax
(backend) in order details, line items states could not be translated

Fixes #4009
[backend] verifying if product picker returns any product

Fixes #4008
do not call move_to_child_with_index when the taxon is new, since this
is not supported by acts_as_nested_set.

Fixes #3999
Show log entries in the admin backend

Working towards a solution for #3978
[backend] Fix issue where database cleaner was not truncating tables
Bump kaminari to 0.15.0