[FIX] mail: add a max limit for chatter

- 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
Please remove the automatic scrolling, this is causing more issues than bringing benefits.