Files which are part of an export, but not owned by the exporting user
are now saved to /files/extra/ in the export tarball.  Ensure that link
rewriting in views, etc., uses this new path.

Change-Id: I91a466a8db83cbaff8fb86a8a58f5fe46b796baa
Signed-off-by: Richard Mansfield <richard.mansfield@catalyst.net.nz>