[FIX] website: proper override of auth_signup

Add the dependency between website and auth_signup
auth_signup is in auto-install but there was no strict dependency

Before this commit, installing website transformed the field
auth_signup_uninvited from a static list to a computed list (due to related).

This fixes the following bug:
1. install auth_signup
-> create selections
2. install webstite
-> remove selections and turn it into a computed field with dynamic
selections
3. update auth_signup
-> recreate the selections when updating the module
4. reload the registry (automatic at the end of update)
-> try to remove the selections -> error as not in install/uninstall
mode

Instead, keep the list static and turn it into a computed field with
inverse

closes odoo/odoo#35861

Signed-off-by: Martin Trigaux (mat) <mat@odoo.com>