PATCH /backlog_items/{id}/children- "remove" and "add" properties

Include:
- refactoring of the way add/remove are validated
- first step of link/unlink element refactoring (artifact link updater)

Part of story #7511 PlanningV2 - Drag'nDrop backlog items

Change-Id: I6e0ec9f2ec5c378cdb39309033f4acfd9f04fe7f
Merge commit 'refs/changes/26/3326/5' of ssh://gerrit.tuleap.net:29418/tuleap into HEAD

* ssh://gerrit.tuleap.net:29418/tuleap:
  PATCH /backlog_items/{id}/children- "remove" and "add" properties

Change-Id: I5075412fcdb9cc55562984407b48d1ab96f12db9
Integrated in Tuleap 7.7.99.59

Change-Id: Id1123d2d77cc1f6fe829b9922d11e08e6acb3aef