[ADD] URL shortener

TASK : https://www.odoo.com/web#id=8277&view_type=form&model=project.task&menu_id=3942&action=327
Note: if the body is void, res[mass_mailing.id] won't be computed and self.convert_link(cr, uid, [mailing.id], context=context)[mailing.id] hereabove will crash ;) .