On tracker:
Add parent tracker info in /tracker/:id route

On milestone:
"resources" : {
        "milestones" : {
            "uri" : "/milestones/666/milestones",
            "accept" : {"trackers" : [{"id" : "21", "uri" : "/trackers/21"}], ...}
        },
        "backlog" : {
            "uri" : "/milestones/666/backlog",
            "accept" : {"trackers": [{"id" : "41", "uri" : "/trackers/41"}, {"id" : "42", "uri" : "/trackers/42"}], ...}
        },
        "content" : {
            "uri" : "/milestones/666/content",
            "accept" : {"trackers": [{"id" : "41", "uri" : "/trackers/41"}, {"id" : "42", "uri" : "/trackers/42"}], ...}
        },
        "cardwall" : "/milestones/666/cardwall", // we could imagine "valid_types" here to list the trackers of the cards that can be in the cardwall
        "burndown" : null
    }
}