Add open/close information in milestone resource

Impacted routes:
/projects/:id/milestones
/milestones/:id
/milestones/:id/milestones

This is part of story #5984 - see open/closed cards percentage fallback

Change-Id: I27d4c70ecfd50c46d78d944bfb1b788e6e733087
Merge commit 'refs/changes/60/1860/2' of http://gerrit.tuleap.net/tuleap into HEAD

* http://gerrit.tuleap.net/tuleap:
  Add open/close information in milestone resource

Change-Id: I0e9607a992bd0009f7f28b0ae29151a327c8df60
This is Tuleap 6.10.99.8

Change-Id: I46b010f9ddbaa52d0caab36d2f66a54598a3ffdd