user upgrades: add bank payment methods for European countries.

Add the following bank redirect payment methods:

* https://stripe.com/docs/payments/bancontact
* https://stripe.com/docs/payments/eps
* https://stripe.com/docs/payments/giropay
* https://stripe.com/docs/payments/ideal
* https://stripe.com/docs/payments/p24

These methods are used in Austria, Belgium, Germany, the Netherlands,
and Poland.

These methods require payments to be denominated in EUR, which means we
have to set prices in both USD and EUR, and we have to automatically
detect which currency to use based on the user's country. We also have
to automatically detect which payment methods to offer based on the
user's country. We do this by using Cloudflare's CF-IPCountry header to
geolocate the user's country.

This also switches to using prices and products defined in Stripe
instead of generated on-the-fly when creating the checkout.

app/models/user_upgrade.rb

@@ -76,32 +76,6 @@ class UserUpgrade < ApplicationRecord
     end
   end
 
-  def upgrade_price
-    case upgrade_type
-    when "gold"
-      UserUpgrade.gold_price
-    when "platinum"
-      UserUpgrade.platinum_price
-    when "gold_to_platinum"
-      UserUpgrade.gold_to_platinum_price
-    else
-      raise NotImplementedError
-    end
-  end
-
-  def upgrade_description
-    case upgrade_type
-    when "gold"
-      "Upgrade to Gold"
-    when "platinum"
-      "Upgrade to Platinum"
-    when "gold_to_platinum"
-      "Upgrade Gold to Platinum"
-    else
-      raise NotImplementedError
-    end
-  end
-
   def level_string
     User.level_string(level)
   end
@@ -178,24 +152,25 @@ class UserUpgrade < ApplicationRecord
   end
 
   concerning :StripeMethods do
-    def create_checkout!
+    def create_checkout!(country: "US")
+      methods = payment_method_types(country)
+      currency = preferred_currency(country)
+      price_id = upgrade_price_id(currency)
+
       checkout = Stripe::Checkout::Session.create(
         mode: "payment",
         success_url: Routes.user_upgrade_url(self),
         cancel_url: Routes.new_user_upgrade_url(user_id: recipient.id),
         client_reference_id: "user_upgrade_#{id}",
         customer_email: purchaser.email_address&.address,
-        payment_method_types: ["card"],
+        payment_method_types: methods,
         line_items: [{
-          price_data: {
-            unit_amount: upgrade_price,
-            currency: "usd",
-            product_data: {
-              name: upgrade_description,
-            },
-          },
+          price: price_id,
           quantity: 1,
         }],
+        discounts: [{
+          coupon: promotion_discount_id,
+        }],
         metadata: {
           user_upgrade_id: id,
           purchaser_id: purchaser.id,
@@ -203,6 +178,7 @@ class UserUpgrade < ApplicationRecord
           purchaser_name: purchaser.name,
           recipient_name: recipient.name,
           upgrade_type: upgrade_type,
+          country: country,
           is_gift: is_gift?,
           level: level,
         },
@@ -252,6 +228,63 @@ class UserUpgrade < ApplicationRecord
       !pending?
     end
 
+    def promotion_discount_id
+      if Danbooru.config.is_promotion?
+        Danbooru.config.stripe_promotion_discount_id
+      end
+    end
+
+    def upgrade_price_id(currency)
+      case [upgrade_type, currency]
+      when ["gold", "usd"]
+        Danbooru.config.stripe_gold_usd_price_id
+      when ["gold", "eur"]
+        Danbooru.config.stripe_gold_eur_price_id
+      when ["platinum", "usd"]
+        Danbooru.config.stripe_platinum_usd_price_id
+      when ["platinum", "eur"]
+        Danbooru.config.stripe_platinum_eur_price_id
+      when ["gold_to_platinum", "usd"]
+        Danbooru.config.stripe_gold_to_platinum_usd_price_id
+      when ["gold_to_platinum", "eur"]
+        Danbooru.config.stripe_gold_to_platinum_eur_price_id
+      else
+        raise NotImplementedError
+      end
+    end
+
+    def payment_method_types(country)
+      case country.to_s.upcase
+      # Austria, https://stripe.com/docs/payments/bancontact
+      when "AT"
+        ["card", "eps"]
+      # Belgium, https://stripe.com/docs/payments/eps
+      when "BE"
+        ["card", "bancontact"]
+      # Germany, https://stripe.com/docs/payments/giropay
+      when "DE"
+        ["card", "giropay"]
+      # Netherlands, https://stripe.com/docs/payments/ideal
+      when "NL"
+        ["card", "ideal"]
+      # Poland, https://stripe.com/docs/payments/p24
+      when "PL"
+        ["card", "p24"]
+      else
+        ["card"]
+      end
+    end
+
+    def preferred_currency(country)
+      case country.to_s.upcase
+      # Austria, Belgium, Germany, Netherlands, Poland
+      when "AT", "BE", "DE", "NL", "PL"
+        "eur"
+      else
+        "usd"
+      end
+    end
+
     class_methods do
       def register_webhook
         webhook = Stripe::WebhookEndpoint.create({