[ADD] URL shortener

TASK : https://www.odoo.com/web#id=8277&view_type=form&model=project.task&menu_id=3942&action=327
This method will be deleted and replaced by a graph view in the other branch of the mass mailing improvement.

See https://github.com/odoo-dev/odoo/blob/master-mass-mailing-queue-jdc/addons/mass_mailing/models/mass_mailing.py