Allowing user exceptions of private ticket restriction

[kgabor@…]

Currently, if a user has none of the introduced new permissions (TICKET_VIEW_*) of PrivateTicketsPlugin, PrivateTicketsPlugin denies access and therefore user has permission denied whether it has the ordinary TICKET_VIEW or not. It would be better to simply do nothing by the plugin if no TICKET_VIEW_* permission present and so it would restore default permission checking for this particular user. In other words, missing of all extra permission would be an exceptional case similar to TRAC_ADMIN in policy.py.

[Noah Kantrowitz]

(In [3751]) If user has none of the special permissions, fall-through to the next policy. Also enforce privatetickets permissions on any sub-resource (eg. attachments) of a ticket. (closes #3097)