Add a default format if no format is defined in the locale file. Use the same default format as the datepicker gets in the translations partial.

Fixes #3602
While an order_id field exists within inventory_units, we should be able to retrieve these using an association

Fixes #3744
Add regression test for #3744
Associate user in Order `merge!` when possible

Users might be losing their cart changes when they come back but don't
visit either the /cart page or the /checkout

Fixes #3742
Fixes #3736
Bump active merchant to 1.38.1

Fixes #3745
refactored spree stock estimator

Extracted functionality out of #shipping_rates for choosing the default
shipping rate and the for the order shipping rates should be sorted in,
in the interest of overriding said behavior.

Change-Id: Iefcce00f1f2ebfedd565dd3aeb540a6111ffc5b7

Fixes #3748
Fix product paging issue

Fixes #2851
Fixes #3064

Conflicts:
	frontend/app/controllers/spree/taxons_controller.rb
Fix expectations in order/adjustments_spec so that it now passes