This moves a step closer to having a simple and straightforward way to
configure the app's mail delivery which doesn't require to be a nuclear
engineer to troubleshoot mail issues.
It happens way too often that servers have mail config broken when
restarted or redeployed and it takes too much brain power to fix it. No
doubt; it's way too complex.
I chose to leave this page's form fields but "Send mails as" as
read-only. This other field is still used by instance manager to
troubleshoot mail issues.
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.
Fixes the following deprecation warning:
```
DEPRECATION WARNING: The configuration option `config.serve_static_assets` has been renamed to `config.serve_static_files` to clarify its role (it merely enables serving everything in the `public` folder and is unrelated to the asset pipeline). The `serve_
static_assets` alias will be removed in Rails 5.0. Please migrate your configuration files accordingly. (called from block in <top (required)> at /usr/src/app/config/environments/test.rb:13)
```
It doesnt work in rails 5 and apparently it was introduced to debug already deleted products cache in staging
See this commit that introduced it 10a79d5a65
Spree used to give you more options to configure ActionMailer but our
setup is much simpler. We can remove unused code.
The removed option was never used by OFN and defaulted to true. We don't
need a database migration because the value isn't set in the database.
