Optimize touch_taxons on product.

This will touch all of the taxons and taxon ancestors for  a product
using a constant number of updates (2, one for taxons, one for
taxonomies).

Previously the number of updates would be 2 * the number of taxons &
taxon ancestors, as it would update the taxonmy for each taxon on each
update.

Fixes #4636
[api] Add ability to assign user_id through Orders#update

Hi, my name is associate_user. You might remember me from smash comedies like 'CheckoutsController'.
[api] Return billing address in user data

Useful for when you want to find a user by their email address, and have their address information display on a page

Like this one http://cl.ly/image/3S1P0W2g0W42
Add shipments API back under orders
CreditCard#expiry= used to assume the incoming format was that which was enforced by Frontend's JS.

No longer true.

Fixes #4429
Fixes #4653
Load a StockItem's variant, even if it's been soft deleted

Alternative patch for #4652

Fixes #4651
Fixes #4652

Conflicts:
	core/spec/models/spree/stock_item_spec.rb
Remove unnecessary test for order.products
Use font-awesome-rails for the icons
[frontend] Replace all (read: one) occurrence(s) of font-awesome icon usage
[backend] Replace all occurrence(s) of font-awesome icon usage with FontAwesome 4.x styles
Relax font-awesome-rails dependency
Use .fa- prefix within click_icon helper
Give trash icon a sensible name
Fix some click_icon usages in shipments_spec
WIP: New Admin Order Interface

The admin order interface in Spree is one of the most painfully slow parts of Spree, and requires a lot of clicking in order to complete an order.

This is an attempt to fix that by using Backbone to display and manipulate the order's information by talking to Spree's API. So far, it looks like this has sped things up slightly.

There's still quite a lot of work to be done.
Add placeholder templates for ember
Include stock locations in small variant responses

This is necessary for the splitting shipments in the admin backend, but may be useful in other instances
(WIP) Manifest item splitting

Awaiting @jhawthorn's better shipment splitting API changes
Display a legend on the complete template
Add ability to capture/void/whatever a payment