PATCH /milestones/:id/content with 'add' and 'remove'

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

Change-Id: I3e9b7b3c6cf7059db0741bdcad260fc3c4a9a3ae
Merge commit 'refs/changes/23/3323/4' of ssh://gerrit.tuleap.net:29418/tuleap into HEAD

* ssh://gerrit.tuleap.net:29418/tuleap:
  PATCH /milestones/:id/content with 'add' and 'remove'

Change-Id: I4fbcb4c21ebe2b79c2e016ef83e15df0d0b2d6d1
This is Tuleap 7.7.99.58

Change-Id: Iaeba38e706cabb9f1c46aa612ee7b3f6f1c56c4f