upgrades: fix Danbooru.config.user_upgrades_enabled? setting

* Fix it so that if upgrades are disabled, only the payment button is disabled instead of the whole page.
* Fix it so that disabling upgrades with DANBOORU_USER_UPGRADES_ENABLED="false" works.

app/models/user_upgrade.rb

@@ -28,7 +28,7 @@ class UserUpgrade < ApplicationRecord
   scope :self_upgrade, -> { where("recipient_id = purchaser_id") }
 
   def self.enabled?
-    Danbooru.config.user_upgrades_enabled?
+    Danbooru.config.user_upgrades_enabled?.to_s.truthy?
   end
 
   def self.gold_price

app/policies/user_upgrade_policy.rb

@@ -6,7 +6,7 @@ class UserUpgradePolicy < ApplicationPolicy
   end
 
   def new?
-    UserUpgrade.enabled?
+    true
   end
 
   def show?

app/views/user_upgrades/new.html.erb

@@ -107,7 +107,9 @@
             <% end %>
           </td>
           <td>
-            <% if @user_upgrade.purchaser.is_anonymous? %>
+            <% if !UserUpgrade.enabled? %>
+              <%= button_to "Get #{Danbooru.config.canonical_app_name} Gold", user_upgrades_path(user_id: @recipient.id), class: "button-primary", disabled: true %>
+            <% elsif @user_upgrade.purchaser.is_anonymous? %>
               <%= link_to "Get #{Danbooru.config.canonical_app_name} Gold", new_user_path(url: new_user_upgrade_path), class: "button-primary" %>
             <% elsif @user_upgrade.recipient.level <= User::Levels::MEMBER %>
               <%= button_to "Get #{Danbooru.config.canonical_app_name} Gold", user_upgrades_path(user_id: @recipient.id, upgrade_type: "gold", country: params[:country], promo: params[:promo], payment_processor: "authorize_net"), class: "button-primary", remote: true, disable_with: "Redirecting..." %>