tasks #6974: Trackers is able to receive emails

forge_artifacts user is added to /etc/aliases.codendi

To test the feature, follow instructions in the dedicated README, launch
system check events and send an email:

    $root> echo "Blah" | mail -s "Test from root" forge__artifacts

The complete email appears in /var/log/codendi/codendi_syslog

This is part of story #6301: reply to a follow-up comment by email.

Change-Id: Ia5e0c3120611b756dd19dc24271a4183b4e82a04
Merge commit 'refs/changes/42/2542/5' of ssh://gerrit.tuleap.net:29418/tuleap into HEAD

* ssh://gerrit.tuleap.net:29418/tuleap:
  tasks #6974: Trackers is able to receive emails

Change-Id: Ie732792a5f75c287fe3d4326c1a6317b1c4b5a09
This is Tuleap 7.1.99.9

Change-Id: Ie79ad0a2ffbe816d61e423c23c02b373839695f0