FIX: Introduce Guardian::BasicUser for oneboxing checks (#24681)

Through internal discussion, it has become clear that
we need a conceptual Guardian user that bridges the
gap between anon users and a logged in forum user with
an absolute baseline level of access to public topics,
which can be used in cases where:

1. Automated systems are running which shouldn't see any
   private data
1. A baseline level of user access is needed

In this case we are fixing the latter; when oneboxing a local
topic, and we are linking to a topic in another category from
the current one, we need to operate off a baseline level of
access, since not all users have access to the same categories,
and we don't want e.g. editing a post with an internal link to
expose sensitive internal information.

lib/discourse.rb

@@ -873,6 +873,10 @@ module Discourse
     @system_users[current_db] ||= User.find_by(id: SYSTEM_USER_ID)
   end
 
+  def self.basic_user
+    Guardian.basic_user
+  end
+
   def self.store
     if SiteSetting.Upload.enable_s3_uploads
       @s3_store_loaded ||= require "file_store/s3_store"