Add Danbooru Winter Sale.

app/views/user_upgrades/new.html.erb

@@ -18,6 +18,10 @@
     <% else %>
       <h1>Upgrade Account</h1>
 
+      <% if Danbooru.config.is_promotion? %>
+        <p><b>Danbooru Winter Sale! Gold and Platinum upgrades are 25% off from now until January 1st.</b></p>
+      <% end %>
+
       <p>Upgrading your account gives you exclusive benefits and helps support
       <%= Danbooru.config.canonical_app_name %>. Your support helps keep the
       site ad-free for everyone!</p>
@@ -44,10 +48,16 @@
           <td></td>
           <td>Free</td>
           <td>
+            <% if Danbooru.config.is_promotion? %>
+              <s>$20</s>
+            <% end %>
             <%= cents_to_usd(UserUpgrade.gold_price) %>
             <div class="fineprint">One time fee</div>
           </td>
           <td>
+            <% if Danbooru.config.is_promotion? %>
+              <s>$40</s>
+            <% end %>
             <%= cents_to_usd(UserUpgrade.platinum_price) %>
             <div class="fineprint">One time fee</div>
           </td>