[FIX] mail: correct activity due date on activity view

Steps to reproduce:
- install crm
- set your computer timezone to GMT-6 (Mexico city)
- go to crm > change the view to activity view

Current behavior:
all activities are one day off from their actual deadline
(an activity set to be due on the 7th of January shows as
6 Jan on the view)

Current behavior:
activities are displayed properly according to their due date

opw-2162579

closes odoo/odoo#43105

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>