upgrades: fix post-upgrade landing page.

* Fix the post-upgrade message showing the gift message instead of the
  normal upgrade message.

* Link back to Danbooru so people don't stay on Safebooru and get
  confused when they can't see hidden posts.

app/controllers/user_upgrades_controller.rb

@@ -55,6 +55,10 @@ class UserUpgradesController < ApplicationController
       flash[:error] = e.message
     end
 
-    redirect_to user_upgrade_path(user_id: params[:user_id])
+    if @user == CurrentUser.user
+      redirect_to user_upgrade_path
+    else
+      redirect_to user_upgrade_path(user_id: params[:user_id])
+    end
   end
 end