Add GET, OPTIONS and PUT routes for milestones/:id/backlog

Change-Id: I15c17a35bd1b7cc5bbd25250277261b930204b25
Merge commit 'refs/changes/21/1521/9' of ssh://gerrit.tuleap.net:29418/tuleap into stable

* ssh://gerrit.tuleap.net:29418/tuleap:
  Add GET, OPTIONS and PUT routes for milestones/:id/backlog

Change-Id: Id1a5b8abe98c79346b7bf275a84b0b633871ef46
This is Tuleap 6.7.99.21

Change-Id: I37b8e3a67c9a2be4d39d7720ccf9ea5fc6795c32