[FIX] website: compute company id for new users

When a new user is created from the website, the company id was always
set to the first company of the database even if the website was the one
of another company. This flow has been already fixed if there is the
"Specific User Account" setting activated (see [this other commit]).
This commit fixes the same issue but for every case.

Steps to reproduce the issue:
- Create 2 companies A & B
- For each company, create a website linked to a different URL
- Activate 'Free sign up' for company B
- As a public user, go to website of company B
- Go to 'Sign in > Don't have an account?' and create an account

=> If as an admin you check the company of the created user, it is
company A instead of company B.

[this other commit]: https://github.com/odoo/odoo/commit/77c708c516beb322df37220634e178ba82e894c9



task-3277317

closes odoo/odoo#120475

Signed-off-by: Benoit Socias (bso) <bso@odoo.com>