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

app/views/user_upgrades/show.html.erb

@@ -3,13 +3,13 @@
     <% if flash[:success] %>
       <h1>Congratulations!</h1>
 
-      <% if params[:user_id] %>
+      <% if user != CurrentUser.user %>
-        <p><%= user.name %> is now a <%= user.level_string %> level account. Thanks for supporting the site!</p>
+        <p><%= user.name %> is now a <%= user.level_string %> user. Thanks for supporting the site!</p>
       <% else %>
-        <p>You are now a <%= user.level_string %> level account. Thanks for supporting the site!</p>
+        <p>You are now a <%= user.level_string %> user. Thanks for supporting the site!</p>
       <% end %>
 
-      <p><%= link_to "Return to posts", posts_path %> </p>
+      <p><%= link_to "Go back to #{Danbooru.config.canonical_app_name}", "https://danbooru.donmai.us" %> to start using your new account.</p>
     <% elsif flash[:error] %>
       <h1>An error occurred!</h1>
       <p><%= flash[:error] %></p>