Fix error "'thumbnail' tag received a bad argument: 'subject_location'"

This happened when I inserted an image into a CMS page:
https://github.com/stefanfoulis/cmsplugin-filer/issues/36

Apparently the configuration of django-filer wasn't completed and needs to be.
The changes made by this patch should fix it in new projects.
Merge pull request #50 from aptivate/fix_thumbnail_processors

Fix error "'thumbnail' tag received a bad argument: 'subject_location'"