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
Use different id on hidden field so associated labels work as expected

Fixes #3981
[backend] verifying if product picker returns any product

Fixes #4008