[contract] related from partner should lead to contract, not analytic account

Task: https://www.odoo.com/web?#id=9518&view_type=form&model=project.task&menu_id=3942&action=327
pad : https://pad.odoo.com/p/r.PiB0ddiS6G1LwQ5X
Please move that definition to a new res_partner.py file.