Extract artifact permissions

This task is part of story #7456 Index artifact permissions

For the artifact permission indexation, we will split in two parts the permissions:
 * tracker
 * artifact

So, to be able to do this, the permission serializer has to serialize each kind of
permission

Change-Id: I87abef1ac294398addff388a43220763c6997b0d
Merge commit 'refs/changes/80/3080/1' of ssh://gerrit.tuleap.net:29418/tuleap into tuleap-stable-master

* ssh://gerrit.tuleap.net:29418/tuleap:
  Extract artifact permissions

Change-Id: I0db4ed97abea3e2117eed983497e5a24c1aabb13
Tuleap 7.5.99.26

Change-Id: I4372a5041c1023a911a2716a8f88c24594b697dd