[FIX] website: correctly set and sync custom account setting

A while back, the behavior of this setting was changed from a
related field to a computed as a fix to various issues with: cb9f48d3
The setting's value synchronization with what is displayed on the
setting's page is broken, since its behavior changed with commit:
2ccc7352 . It resets to its default value for the default website when
editing a secondary one and is not only confusing but breaks the
setting in some scenarios.

When changing the auth_signup_uninvited setting, the changes
are correctly reflected and not reset to default upon changing
the website we are currently editing.

task-2612686

closes odoo/odoo#76262

Signed-off-by: Romain Derie (rde) <rde@odoo.com>