Add Danbooru Winter Sale.

* 25% off Gold and Platinum accounts.
* Free 6 tag searches for Member-level users.
* Runs from midnight Dec. 25th to 28th (UTC time).

app/logical/user_upgrade.rb

@@ -1,13 +1,25 @@
 class UserUpgrade
   def self.gold_price
-    2000
+    if Danbooru.config.is_promotion?
+      1500
+    else
+      2000
+    end
   end
 
   def self.platinum_price
-    4000
+    if Danbooru.config.is_promotion?
+      3000
+    else
+      4000
+    end
   end
 
   def self.upgrade_price
-    2000
+    if Danbooru.config.is_promotion?
+      1500
+    else
+      2000
+    end
   end
 end