Fix translation by using ActiveRecord's default

Our translations are not available when decorators are loaded. The
message for a missing product category was missing:
https://github.com/openfoodfoundation/openfoodnetwork/issues/1829

I moved the translation to ActiveRecord's default scope so that it can
be picked up automatically.

app/models/spree/product_decorator.rb

@@ -25,7 +25,7 @@ Spree::Product.class_eval do
 
   # validates_presence_of :variants, unless: :new_record?, message: "Product must have at least one variant"
   validates_presence_of :supplier
-  validates :primary_taxon, presence: { message: I18n.t("validation_msg_product_category_cant_be_blank") }
+  validates :primary_taxon, presence: true
   validates :tax_category_id, presence: { message: I18n.t("validation_msg_tax") }, if: "Spree::Config.products_require_tax_category"
 
   validates_presence_of :variant_unit

config/locales/en.yml

@@ -34,6 +34,8 @@ en:
         email: Customer E-Mail
       spree/payment:
         amount: Amount
+      spree/product:
+        primary_taxon: Product Category
       order_cycle:
         orders_close_at: Close date
     errors:
@@ -2392,7 +2394,6 @@ See the %{link} to find out more about %{sitename}'s features and to start using
   enterprise_fee_by: "%{type} fee by %{role} %{enterprise_name}"
   validation_msg_relationship_already_established: "^That relationship is already established."
   validation_msg_at_least_one_hub: "^At least one hub must be selected"
-  validation_msg_product_category_cant_be_blank: "^Product Category cant be blank"
   validation_msg_tax: "^Tax Category is required"
   validation_msg_tax_category_cant_be_blank: "^Tax Category can't be blank"
   validation_msg_is_associated_with_an_exising_customer: "is associated with an existing customer"