Revert "Temp disable account upgrades."

This reverts commit c4cdba0874.

app/controllers/user_upgrades_controller.rb

@@ -4,8 +4,6 @@ class UserUpgradesController < ApplicationController
   skip_before_action :verify_authenticity_token, only: [:create]
 
   def create
-    raise NotImplementedError
-
     if params[:stripeToken]
       create_stripe
     end

app/views/user_upgrades/new.html.erb

@@ -92,16 +92,14 @@
     <% end %>
 
     <div class="section">
-      <% if true %>
-        <p>Account upgrades are temporarily disabled. Upgrades will be available again after December 1st.</p>
-      <% elsif params[:user_id] %>
+      <% if params[:user_id] %>
         <p>You are gifting this account upgrade to <%= link_to user.pretty_name, user_path(params[:user_id]) %>.</p>
       <% else %>
         <p>You can also upgrade someone else's account for the same price. The easiest way is to go to their profile page and look for a "Gift Upgrade" link.</p>
       <% end %>
     </div>
 
-    <% if false %>
+    <% if Danbooru.config.stripe_publishable_key %>
       <% if CurrentUser.is_anonymous? %>
         <p><%= link_to "Sign up", new_user_path %> or <%= link_to "login", new_session_path(url: new_user_upgrade_path) %> first to upgrade your account.</p>
       <% elsif CurrentUser.safe_mode? %>