[FIX] mail: limit messages in chatters

- Task: https://www.odoo.com/web#id=6729&view_type=form&model=project.task&action=333&active_id=131
- Pad: https://pad.odoo.com/p/r.q6QePBp6BFlNfBEg

(This PR supersedes work done in #782 #2597)

 @mart-e It could interest you ;-)