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/models/user.rb

@@ -505,6 +505,8 @@ class User < ApplicationRecord
         Danbooru.config.base_tag_query_limit * 2
       elsif is_gold?
         Danbooru.config.base_tag_query_limit
+      elsif is_member? && Danbooru.config.is_promotion?
+        Danbooru.config.base_tag_query_limit
       else
         2
       end