[FIX] website: sign-up in multi-website

- Create 2 companies A & B
- For each company, create a website linked to a different URL and
  activate 'Free sign up' and 'Specific User Account'.
- As a public user, go to website of company B
- Go to 'Sign in > Don't have an account?' and create an account

The user is created in company A (the company of the user template)
instead of company B (the company of the website).

We set the company of the user based on the company of the website.

opw-1962197

closes odoo/odoo#32823

Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>