[CRM] Clean Business Model

- Task: https://www.odoo.com/web#id=8862&view_type=form&model=project.task&menu_id=3942&action=327
- Pad: https://pad.odoo.com/p/r.bMVjkCKLyLgbuEzl
Please rename the m2m table also, while you are at it. Moreover I don't think category_id field exists on the crm.lead.tag model if I am not wrong.