Refactor Milestone to extract MilestonePlan.

A MilestonePlan is the Right pane of MilestonePlanning view
It allows to save some more ticks (therefore the forest) by:
- avoiding a fetch of PlannedArtifacts on Burndown view (useless)
- computation of remaining effort & capacity on cardwall & burndown view
  (might be really time consuming with computed fields)
Futhermore, it will allow to fetch things smartly on cardwall in the future.

Change-Id: I25ec09e6f21b5f0b07c1103d719bfcfbef86fe2e
Merge commit 'refs/changes/64/264/3' of http://gerrit.tuleap.net/tuleap into tuleap-stable-master

* http://gerrit.tuleap.net/tuleap:
  Refactor Milestone to extract MilestonePlan.

Conflicts:
	plugins/tracker/tests/Tracker_Artifact_ChangesetValue_OpenListTest.php
Tuleap 5.10.99.25