Fix shipping methods showing up multiple times in admin
Switch the mailers to be invoked using ids, not model objects. This makes it easier to support asynchronous mail jobs, as only simple ids need to be persisted.

Fixes #2808
Better factory integration for extensions.

Related to #2780
Update extension README, detailing new factories integration.

Related to #2780
Find visible select2-input element within promotion_adjustments_spec
Select correct element in order_details_spec to alter the quantity of

This test was working for SQLite, but in MySQL+PostgreSQL the order was different, causing the test to break.
Target correct element in stock_managment_spec

This test worked in SQLite+MySQL, but not in PostgreSQL. This commit fixes that.
Fix incorrect syntax in products/_thumbnails for @product.variant_images
Fix issue where two 'Clothing' taxons were being created in taxons_spec