This is a follow-up of MDL-58823

Right now, repositories that supports links to external files are doing the redirections to those files using header('Location, this is not ideal, they should use the redirect() function

The problem is that the redirect() function is not compatible with webservice/pluginfile.php because this php script is defined as an AJAX_SCRIPT (so on any redirection, the redirect function will trigger an exception)

Ideally we should:

• Use redirect in repositories
• Remove the AJAX_SCRIPT from webservice/pluginfile.php but at the same time supporting JSON exceptions (that's the reason why is defined in that way)