[FIX] mail: activity view: luxon in rendering context

Since the refactoring of the activity view, luxon is not in the
rendering context. Resulting in crashing in some activity views
that used luxon in their template.

- Go to Appraisal
- Switch to the activity view
- Create a record in this view -> crash

Now, luxon is in the rendering context.

closes odoo/odoo#109153

Signed-off-by: Luca Vitali <luvi@odoo.com>