The concept of 'parent_id' on website.visitors was introduced in the saas-13.3
stable while implementing the "event online" feature:
However, it should have been part of the website module from the start since
it's a 'global' concept that does not depend on events at all.
See #53540 for more details.
This commit aims to clean the code by moving the field to the website module,
which allows a nice cleaning of associated overridden methods as well.
Along with that, we move the website.visitor demo data from the event module to
the website module, allowing a fresh install of website to showcase some of our
visitors feature.
We also took this opportunity to do some minor improvements in the visitors
kanban view in order to make relevant information more visible.
Task-2429652
Part-of: odoo/odoo#65113
Aurélien Warnon
0641cb20e8