[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
More globally, the tooltip style should be close to others tooltips used in Odoo (see http://getbootstrap.com/javascript/#tooltips)

By the way, why did you use popovers and not tooltips?