listing_spec: use active_support date functions

This could fail if run in the evening when running specs in timezones
with negative offsets.

Using Time.current and Date.current ensures values entered are in UTC.

Fixes #4038 AND probably the CI build.
Use flash.now for coupon errors

Fixes #4034
[backend] nil check for shipping_method in orders/_shipment

fixes #4025
Show destroy flash messages in admin backend

Fixes #4032
Really always include the confirm step preference

Fixes #3945

Conflicts:
	frontend/spec/features/checkout_spec.rb
Don't show confirmation step if payments are empty
[frontend] Set flash.now for gateway errors in CheckoutController

Fixes #4034
[frontend] Use flash.now for coupon application error in OrdersController

Otherwise this will carry over to the next request, which is not fun.

Fixes #4034
Use new show_flash erorr in payments/edit.js.coffee