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

Change-Id: Ic94c39147df048598d42596a9efd72fd3d47908f
Merge remote-tracking branch 'gerrit/5579_project_backlog' into HEAD

* gerrit/5579_project_backlog:
  Add GET, OPTIONS and PUT routes for projects/:id/backlog

Change-Id: I1d5aabad79c6fa0bc445c9de3219ccf111882d4c
This is Tuleap 6.7.99.26

Change-Id: Id427a8686a32042f4d0f8d6fda2a1dae796f03ae