[#6608] Close milestones on import if no open tickets

Signed-off-by: Tim Van Steenburgh <tvansteenburgh@gmail.com>
[#6608] Trac ticket import cleanups

* Set default Trac open/closed statuses
* If a ticket urlopen call fails during import, let the import fail
  rather than trying to continue in an unknown state
* Add test for setting milestone status during import

Signed-off-by: Tim Van Steenburgh <tvansteenburgh@gmail.com>