Refactor settings model

Move the various sections to their own concerns, so they're easier to
find and maintain

app/models/concerns/settings/btcpay_settings.rb

@@ -0,0 +1,24 @@
+module Settings
+  module BtcpaySettings
+    extend ActiveSupport::Concern
+
+    included do
+      field :btcpay_api_url, type: :string,
+        default: ENV["BTCPAY_API_URL"].presence
+
+      field :btcpay_enabled, type: :boolean,
+        default: ENV["BTCPAY_API_URL"].present?
+
+      field :btcpay_public_url, type: :string,
+        default: ENV["BTCPAY_PUBLIC_URL"].presence
+
+      field :btcpay_store_id, type: :string,
+        default: ENV["BTCPAY_STORE_ID"].presence
+
+      field :btcpay_auth_token, type: :string,
+        default: ENV["BTCPAY_AUTH_TOKEN"].presence
+
+      field :btcpay_publish_wallet_balances, type: :boolean, default: true
+    end
+  end
+end