Use human_attribute_name for promo calculator labels

Promotion calculators have preferences, and these need to be translated
with the correct scope.

When you specify a preference for a calculator, a virtual attribute is
generated. For example:

```
class MyCalculator < Spree::Calculator
  preference :percent, :integer, default: 10
end
```

In this scenario, we get a virtual attribute `preferred_percent` on the
`MyCalculator` class. The expectation is that then one can translated
that attribute by calling:
`MyCalculator.human_attribute_name(:preferred_percent)`, and - crucially
- that this attribute is used in form labels.

While all other calculator forms use this (through `form.label
:preferred_percent`) or similar, the default form did not, and instead
used `I18n.t(:percent, scope: :spree)`, which is not specific to the
calculator, and might lead to mistranslations.

Author: mamhoff

promotions/config/locales/en.yml

@@ -311,6 +311,7 @@ en:
         preferred_currency: Currency
       solidus_promotions/calculators/percent:
         description: Provides a discount calculated by percent of the discountable amount of the item being discounted
+        preferred_percent: Percent
       solidus_promotions/calculators/distributed_amount:
         description: Distributes the configured amount among all eligible line items of the order.
         explanation: |
@@ -350,6 +351,7 @@ en:
             </tbody>
           </table>
         preferred_amount: Amount
+        preferred_currency: Currency
 
     errors:
       models:

promotions/lib/views/backend/solidus_promotions/admin/calculator_fields/_default_fields.html.erb

@@ -2,5 +2,5 @@
   <%= render "solidus_promotions/admin/shared/preference_fields/#{calculator.preference_type(name)}",
     name: "#{prefix}[calculator_attributes][preferred_#{name}]",
     value: calculator.get_preference(name),
-    label: t(name.to_s, scope: 'spree', default: name.to_s.humanize) %>
+    label: calculator.class.human_attribute_name("preferred_#{name}", default: I18n.t(name, scope: :spree, default: name.humanize)) %>
 <% end %>