Add REST route to reorder children

PATCH /backlog_items/:id/children
{
  "order": {
    "ids": [123, 456],
    "compared_to": 789,
    "direction": "after"
  }
}

Part of story #7511: Drag'nDrop backlog items

Change-Id: Iecb5a69e78583771138830fc69c0edf901e45ae6
Merge commit 'refs/changes/61/3261/8' of ssh://gerrit.tuleap.net:29418/tuleap into tuleap-stable-master

* ssh://gerrit.tuleap.net:29418/tuleap:
  Add REST route to reorder children

Conflicts:
	plugins/agiledashboard/include/autoload.php

Change-Id: Ie1cad547726b1d0af00e86c57c1c5a472140a50a
This is Tuleap 7.7.99.41

Change-Id: I759b54f4e293427b5cfed4f0570d26167de3a38d