[FEATURE] Multiple Shipments

Status
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**READY**


Description
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Re-factor the Single Page Checkout to reflect more than one shipment in a single order. Allows for selection of shipping options for each shipment. Also, a couple of small bug fixes:
 - Make the `state_name` input box in the address form reflect the correct state if an address already exists for the order
 - Add a 'Updating your order' notification to the Order summary while an API call is in progress

Purpose
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- [x] Add Handlebars.js
- [x] Include a Handlebars template for shipments
- [x] Re-factor js to use Handlebars template instead of manually adding elements to the DOM
- [x] Add 'order updating' notification
- [x] Fix `state_name` input field


Deploy Notes
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N/A