[imp]mail: when mouse over on like button tooltip show users name who li...

...ked message

 - 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
I would add a limit parameter; if there are 200 voters we won't have the place to display it. Maybe display the 10 first voters, and then 'and XX more' in chatter.