[IMP] website*: use published mixin rather than own field

*website_crm_partner_assign, website_customer

Before this commit, some models would create their own `website_published`
field rather than using the `website.published.mixin` mixin which should be
used instead.

That was done to be able to override the default value of the boolean.

This commit simply adds the possibility to override the default value of the
mixin field.

Also, help and string could simply be added on xml views rather than overriding
the field.