Refactor assignee retrival

Change-Id: I51c2d533ead075e17529a1b462bbe4bfac7afd2e
tasks #7442 - Permissions: get artifact permissions usable by ES

Serialize tracker permissions

Given permissions set on tracker, return the list of user groups granted to
access the artifact

Change-Id: I01ef9d3a5af28eb9d122ff6d073c09d185d84861
Merge commit 'refs/changes/05/3005/3' of ssh://gerrit.tuleap.net:29418/tuleap into stable

* ssh://gerrit.tuleap.net:29418/tuleap:
  tasks #7442 - Permissions: get artifact permissions usable by ES
  Refactor assignee retrival

Conflicts:
	plugins/tracker/include/autoload.php
This is Tuleap 7.5.99.18

Change-Id: Ieb45eae8184bc15d6e1eb2f2088001f4a7bf1a8e