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/javascript/src/javascripts/common.js

@@ -8,6 +8,13 @@ $(function() {
     e.preventDefault();
   });
 
+  $("#hide-promotion-notice").on("click.danbooru", function(e) {
+    $("#promotion-notice").hide();
+    Cookie.put('hide_promotion_notice', '1', 7);
+    Cookie.put('hide_upgrade_account_notice', '1', 7);
+    e.preventDefault();
+  });
+
   $("#hide-dmail-notice").on("click.danbooru", function(e) {
     var $dmail_notice = $("#dmail-notice");
     $dmail_notice.hide();