Fix whitespace
Refactor Spree::Adjustment spec to have explicit describe blocks

* Rather than naming the block scopes, which is pretty generic, I
  changed it to an explicit method name, which sets the stage for
  the specs for .manual in a future commit
* Use the outer adjustment method instead of creating a special one
  inside the describe block.
Refactor adjustment sum methods to use aggregate calculation

* The previous code was retrieving the objects from the database and
  them summing the amounts in-memory. A more efficient approach is
  to use the sum method to offload calculation to the database
  engine.
Add Spree::Adjustment.manual scope

* I was unsure what to use as a name for an adjustment that has no
  originator. I was thinking possibly anonymous, or miscellaneous,
  but I thought perhaps manual was a better name because it's more
  clear that it is the result of a human initiated action and not
  generated by the system. I'm open to better suggestions though.
Add Spree::Order#manual_adjustment_total

* This method will sum the eligible manual adjustments for the
  order.
* Change specs to use describe instead of context since they are
  describing specific method behaviour.
Add Spree::Order#discount_total

* This method is responsible for summing the adjustments related to
  discounts. It will be used in Spree::Payment to calculate the
  discount total to send to the payment gateway.
Change payment processing to use Spree::Order#discount_total

* In most cases discount_total and promo_total will be the same
  value, except when there are manual adjustments made to an order.

Related to #3772
Fix shipping methods sample data

We were association a not shipping calculator to the shipping method.
Users trying to edit the samples shipping method would get a Shipping
Method not found because it was associated with a calculator not
available.

Fixes #3776