[imp] mail: display who likes a post

 - Task: https://www.odoo.com/web#id=8679&view_type=form&model=project.task&action=333&active_id=131
 - Pad: https://pad.odoo.com/p/r.VlbOhsbzhWbR4Cga
See this note, still not fixed https://github.com/odoo/odoo/pull/1729#discussion_r16109187