(#3416) Allow site files to be exported by users too.
This change involves relaxing the check that artefacts can only be exported if they're owned by the user (naturally, site files are not). Unfortunately, there's no easy replacement check that I can drop in to continue to guarantee security. $user->can_view_artefact has two problems: doesn't respect site files, and is slow. I added a commented out hypothetical permission checking block and a comment as to why it wasn't uncommented. Signed-off-by: Nigel McNie <email@example.com>