[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
This will trigger a request on every hovering of the button. I'd suggest you cache the result of the first request into the DOM (with a `data-` attribute), so that we reduce the number of requests. 

(I know this is not perfect if someone else likes in the meantime, but this sounds like an acceptable compromise.)