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