[REF] website_crm_partner_assign : use stored editable r+w fields

PURPOSE

Try to move from onchange / default_get to stored editable computed fields.
Behavior should be the same (computed or set by user), with support of
create / write / onchange field update without additional code.

SPECIFICATIONS

Update classic fields updated in some cases by onchange and/or default methods
by fields with store=True, readonly=False. It means their value comes either
from manual user input, either from trigger based coputation.

Remove onchange and default_get when possible, leading to an unique computation
method and clearing fields definition.

Also clean some fields definition inconsistencies, notably required fields
that should instead be correctly computed or default that have no real meaning.

SPECIFICATIONS: FIELDS RENAMING

In this commit we also rename ``date_assign`` to ``date_partner_assign``. There
are already ``date_open`` (crm_lead) and ``assign_date`` (website scoring)
fields available. It may be confusing. The field is therefore renamed to better
indicate it is linked to partner assignment.

LINKS

Task ID 2088565 (crm: from onchange to compute)
Upgrade PR odoo/upgrade#781