[ADD] website: new countdown widget

- Task: https://www.odoo.com/web#id=8287&view_type=form&model=project.task
- Pad: https://pad.odoo.com/p/r.0coHaMolBpsxrIkg
@pga-odoo What @xmo-odoo means, is that sometimes, second could be == 60 https://en.wikipedia.org/wiki/Leap_second