Calculate tax total correctly which depends on the item that could be an Order or a LineItem. Tax adjustments are stored differently in these two cases.

Fixes #4830
Fixes #4829
Speed up rename_adjustment_fields migration

I went to perform this migration on our production db while our store was in maintenance mode and realized it was going to take many hours -- it was loading some million or so Order objects, one at a time.

The first change is that we go from loading all Adjustments and asking whether their adjustable is an Order to just scoping the Adjustment call to only ask for the ones that are. That saved us about one million `if` statements.

The second change is to simply copy the `adjustable_id` over to `order_id` without ever fetching the Order -- who needs it? This saved us about one million ActiveRecord queries.

Happy to pass the fruits along to others on the 2.1--2.2 upgrade path!

Fixes #5121
Make Shipment.cost default to 0

Since upgrading to Spree 2.2, I've noticed exceptions getting thrown by, for instance [`cost + promo_cost`](https://github.com/spree/spree/blob/2-2-stable/core/app/models/spree/shipment.rb#L151) throwing `NoMethodError: undefined method `+' for nil:NilClass`.

Looking into my db schema, I see that virtually all decimal figures in Spree default to 0, but it looks like for some reason `spree_shipments.cost` didn't get that default. This migration helps ensure a smooth upgrade path from earlier Spree releases to 2.2.

Fixes #5128
Dont advance order state when adding an item to cart

As of d02f67dbbf7e8ae1f61b65066c2a3a8234796a11

Admin would click to add a item to the order which would create the line
item and shipment but because we advance the order to address state the
shipments would be destroyed Order#ensure_available_shipping_rates

Cant see why we needed advance order in the first place.
Add completed back as default state.
Update Google Analytics

Updates GA tracking code to support Display Advertising. See: https://support.google.com/analytics/answer/2444872 for more.

Fixes #5129
Allow assigning shipped_at to shipments.

Conflicts:
	core/spec/lib/spree/core/importer/order_spec.rb