ADD: ref_urls now has the option of allowing unauthorized access.

* This can be used to speed up thumbnail retrieval on slow servers.
* You can configure which images are allowed to be accessed without
  being logged in.


git-svn-id: http://svn.montala.net/svn/resourcespace@5055 c08608d7-6e46-0410-86ca-f2a6f1370df5