Fixes#6435 i.e. If the customer paid for their order by Stripe/Paypal then the Enterprise needs to know that the order was cancelled in order to arrange a refund. Refunds are not automatically processed when an order is cancelled.
This will send a very basic email to the shop, it only includes a link to view the cancelled order in the admin area initially.
I created a CustomerOrderCancellation object here because orders can be cancelled in two ways (1) by the customer, so an email should be sent to the shop. (2) by the shop, so an email doesn't need to be sent. However the code for cancelling order happens in Order#cancel via the state machine. Rather than passing some sort of parameter into #cancel to indicate whether it is a customer or shop cancelled order it might be clearer to have a CustomerOrderCancellation object, there could be other differences between customer or shop cancelled orders in future maybe.
Instead of relying on Spree::Order#outstanding_balance we make us of the
result set `balance_value` computed column. So, we ask PostgreSQL to
compute it instead of Ruby and then serialize it from that computed
column. That's a bit faster to compute that way and let's reuse logic.
We hide this new implementation under this features' toggle so it's only
used when enabled. We want hit the old behaviour by default.
This makes it possible to deploy it without releasing it to users since
the toggle is not enabled for anyone.
It aims to make the balance calculation consistent across pages.
This query object is meant to be reusable but those includes are
context-specific and will likely not be needed when reusing the query
elsewhere. If we keep them there, chances are next dev might not notice
it and will introduce a performance regression.
It's simpler and many orders of magnitude more efficient to ask the DB
to aggregate the customer balance based on their orders. It removes
a nasty N+1.
The resulting SQL query is:
```sql
SELECT customers.*, SUM(spree_orders.total - spree_orders.payment_total) AS balance
FROM "customers"
INNER JOIN "spree_orders"
ON "spree_orders"."customer_id" = "customers"."id"
WHERE "customers"."enterprise_id" = 1
AND (completed_at IS NOT NULL)
AND (state != 'canceled')
GROUP BY customers.id
ORDER BY email;
```
I'm not sure why this spec started failing. Fixes:
```
Spree::OrdersController viewing cart when an item is in the cart the page provides the right registration path
Failure/Error: expect(subject.registration_path).to eq registration_path
ActionController::UrlGenerationError:
No route matches {:action=>"index", :controller=>"registration"}
# ./spec/controllers/spree/orders_controller_spec.rb:140:in `block (5 levels) in <top (required)>'
```
