FEATURE: Add support for PLN currency (#258)

Stripe supports the Polish złoty so we can add support for it.

https://meta.discourse.org/t/342040

app/serializers/discourse_subscriptions/payment_serializer.rb

@@ -55,6 +55,8 @@ module DiscourseSubscriptions
         "R"
       when "chf"
         "CHF"
+      when "pln"
+        "zł"
       else
         "$"
       end

assets/javascripts/discourse/controllers/admin-plugins-discourse-subscriptions-products-show-plans-show.js

@@ -44,6 +44,7 @@ export default class AdminPluginsDiscourseSubscriptionsProductsShowPlansShowCont
       { id: "JPY", name: "JPY" },
       { id: "ZAR", name: "ZAR" },
       { id: "CHF", name: "CHF" },
+      { id: "PLN", name: "PLN" },
     ];
   }
 

assets/javascripts/discourse/helpers/format-currency.js

@@ -28,6 +28,9 @@ export function formatCurrency([currency, amount]) {
     case "CHF":
       currencySign = "CHF";
       break;
+    case "PLN":
+      currencySign = "zł";
+      break;
     default:
       currencySign = "$";
   }

config/settings.yml

@@ -36,6 +36,7 @@ discourse_subscriptions:
       - SGD
       - ZAR
       - CHF
+      - PLN
   discourse_subscriptions_campaign_enabled:
     client: true
     default: false