[FIX] calendar: duplicated emails when creating allday meeting

Steps to reproduce:

  - Switch to debug mode
  - Go to calendar
  - Create an allday event in the future named test
  - Add an attendee with an email address
  - Save
  - Go to Settings > Technical > Email > Emails

  Two emails have been sent:
  * Invitation to test
  * test: Date updated

Cause of the issue:

  The method `_inverse_dates` is called when creating an event,
  triggering a write on the start field.
  This then causes an update email to be sent

Solution:

  Add a context key and don't send the update email if it is present

opw-2841276

closes odoo/odoo#100784

X-original-commit: 00591c66
Signed-off-by: Arnaud Joset <arj@odoo.com>
Signed-off-by: Hubert Van De Walle <huvw@odoo.com>