request #7385: Burndown with a big remaining effort tree issues step 1

Allowing tracker admins to choose to store past burndown data in cache.
Note: this only saves their preference

Change-Id: I7400428f47b23b9b148636ca4130f11f2537591a
Merge commit 'refs/changes/02/3102/4' of ssh://gerrit.tuleap.net:29418/tuleap into tuleap-stable-master

* ssh://gerrit.tuleap.net:29418/tuleap:
  request #7385: Burndown with a big remaining effort tree issues step 1

Conflicts:
	plugins/tracker/include/autoload.php

Change-Id: I297bb3e7850928a2f0e2f653b9b3d2cf731acef4
This is Tuleap 7.6.99.38

Change-Id: I177c15e8dd1bd6c1e4dda9597cf7743ce05a6ce6