FIX: check permalinks for deleted topics

- allow to specify 410 vs 404 in Discourse::NotFound exception
- remove unused `permalink_redirect_or_not_found` which
- handle JS side links to topics via Discourse-Xhr-Redirect mechanism

lib/discourse.rb

@@ -77,7 +77,18 @@ module Discourse
   end
 
   # When something they want is not found
-  class NotFound < StandardError; end
+  class NotFound < StandardError
+    attr_reader :status
+    attr_reader :check_permalinks
+    attr_reader :original_path
+
+    def initialize(message = nil, status: 404, check_permalinks: false, original_path: nil)
+      @status = status
+      @check_permalinks = check_permalinks
+      @original_path = original_path
+      super(message)
+    end
+  end
 
   # When a setting is missing
   class SiteSettingMissing < StandardError; end