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.
DEPRECATION WARNING: `#deliver` is deprecated and will be removed in Rails 5. Use `#deliver_now` to deliver immediately or `#deliver_later` to deliver through Active Job.
The way we set up email sending completely changes with Spree 2. This
change encapsulates that code in a single method so that it can be
changed easily and doesn't create further merge conflicts while we are
still working on the master branch and the Spree upgrade.
A bug introduced in a9c37c162e caused the
creation of confirmation emails for shops to fail. The email template
got fixed now and the email address from the database is displayed if
present.
