Hide attachments in history if user can't see them

If user is not allowed to view attachments (i.e. their access level is
lower than $g_view_attachments_threshold), then the history should not
display information about attachments.

Fixes http://www.mantisbt.org/bugs/view.php?id=17744
Maybe not - I just followed the same approach that was previously implemented for other checks; if I change this one I guess for consistency we should fix the others too