[IMP] web : Add Indicator to notify user when connection get lost

Task: https://www.odoo.com/web#id=9963&view_type=form&model=project.task&menu_id=3942&action=327
Pad: https://pad.odoo.com/p/r.ZzhDtxgl8FMoYhXu
@msh-odoo , it seem ok to call rpc every 2 seconds