backupaccount: Don't print page HTML in XML export

The current Atom/XML account backup contains the backupaccount HTML at the end
of the downloaded file. This change makes it so that only the XML is downloaded
by terminating the script before the HTML is served.

actions/backupaccount.php

@@ -74,6 +74,9 @@ class BackupaccountAction extends FormAction
         // @fixme atom feed logic is in getString...
         // but we just want it to output to the outputter.
         $this->raw($stream->getString());
+
+        // Don't print the page HTML
+        exit(0);
     }
 
     public function isReadOnly($args) {