tasks #7367 Add link in notification

This task is part of story #7354: finish story unsubscribe to artifact notification

* The link to unsubscrive is on top of mail (on the right of breadcrumb)
* Adding new object to deal with subscription

Change-Id: I09eaf735db5a3178879854de522cfd733ce73fad
Merge commit 'refs/changes/19/2919/7' of ssh://gerrit.tuleap.net:29418/tuleap into stable

* ssh://gerrit.tuleap.net:29418/tuleap:
  tasks #7367 Add link in notification
This is Tuleap 7.4.99.5

Change-Id: Ibb54cac6a1fbe5ae1d24b92ec684e5deebfd80f0