story #6935: be able to append a backlog item directly to a milestone

Change-Id: I8c19b0241e77877107e41479466d7076c2d75b63
Merge commit 'refs/changes/38/3038/4' of ssh://gerrit.tuleap.net:29418/tuleap into tuleap-stable-master

* ssh://gerrit.tuleap.net:29418/tuleap:
  story #6935: be able to append a backlog item directly to a milestone

Change-Id: I9d850410de2fd8f85be82736ebfe7d9e6d6e2ef0
Tuleap 7.5.99.15

Change-Id: I9e7016f08e87a602d15d2387256acfcbae246fd0