:scissors:
Merge pull request #5324 from camelmasa/refactor-shipment_handler

Refactor Spree::ShipmentHandler
corrects fetch_cart urls
Fixes #5325
removes window.location.origin

isn't compatible with Internet Explorer

Fixes #5323
Revert "Only cancel cancellable shipments."

This reverts commit d870c4e316e3aad6f63c728d224f6a16d4edde57.
Fix form fields.
Bootstrapping Email HTML templates.
Fix email specs.
Add test rake task and refactor layout.
Cleanup unused premailer/ink stuff.
Cleanup templates.
Fix broken spec due to LineItem#options= not being able to assign target_shipment.
Dont create product if master variant invalid.
[Fixes #5306]
Fixes #5330