[FIX] calendar: cannot send mail if current user not in the loop

Steps:
- Log in as admin
- Install calendar
- Activate the developer mode
- Go to Calendar
- Create a new event:
  - Attendee: demo (1)
  - Misc tab:
    - Owner: demo (1)

Bug:
Access Denied:
(Document type: Message, Operation: read) - (Records: [84], User: 2)

Explanation:
The author of the message is not the current user from this commit
onward: https://github.com/odoo/odoo/commit/160d321b821c669185054b7fb8515e88fc22bd5a


The access denial appears when trying to add the ICS file to the
message. The current user is not allowed to read the old attachments
since they are not a part of the discussion.

opw:2454395

closes odoo/odoo#66166

Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>