[IMP] website_event_track: retrieve sponsor website from partner

The Sponsor Website field in event sponsors is now automatically computed from the selected
partner's data, but the field can still be changed afterwards in case
the partner wants to specify a different website url for its sponsoring.

closes odoo/odoo#48217

Task: 2205443
Related: odoo/upgrade#981
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>