PATCH /milestones/:id/backlog

Part of story #7511: Drag'nDrop backlog items

Change-Id: I2f43c6f6e7888f7491fe30b05e08a2aaa4aa408e
story #7511: PATCH /backlog_items/{id}/children should check items belong to {id}

Change-Id: Ib82076eb4fde668230c57bb974f907f92554fec6
Merge commit 'refs/changes/10/3310/7' of ssh://gerrit.tuleap.net:29418/tuleap into stable

* ssh://gerrit.tuleap.net:29418/tuleap:
  story #7511: PATCH /backlog_items/{id}/children should check items belong to {id}
  PATCH /milestones/:id/backlog
This is Tuleap 7.7.99.54

Change-Id: If32f42fbe5dbf68413e26d6d60b33972071d8993