[FIX] website_event: fix ACL issue during registration

Bug
===
Register to an event with a public user.
Sometimes an ACL error will be raised.

Technical
=========
Before flushing the environment (after the creation of the event
registration in the endpoint `registration_confirm`), the ORM will
recompute all needed stored fields. As we didn't write on some of them
(`date_closed`, `utm_campaign_id`, `utm_source_id`, `utm_medium_id`)
they are in the list `Environment::all.tocompute`. So, the ORM will recompute
the fields even if the state didn't change (this behavior can be verify
in `BaseModel::recompute`). This is done after the "create" call of the
event registration and so without the SUDO flag.

Task-2299227

closes odoo/odoo#54624

X-original-commit: 38f37cd483b7907b20171e2c44e14e69e3e00596
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>