[FIX] web_calendar: events not appearing in arabic
[FIX] web_calendar: events not appearing in arabic

Fullcalendar expect as `start` and `end` property of an event object:

> When specifying Event Objects for events or eventSources, you may
> specify a string in IETF format (ex: "Wed, 18 Oct 2009 13:00:00 EST"),
> a string in ISO8601 format (ex: "2009-11-05T13:15:30Z") or a UNIX
> timestamp.

(as seen in https://fullcalendar.io/docs1/event_data/Event_Object/)

Before this commit we could for example in arabic provide an invalid
date string "٢٠١٧-٠٣-١٦ ١٦:٠٠:٠٦" instead of "2017-03-16 16:00:06".

This commit solves this by always providing fullcalendar an ISO8601 date
string.

opw-705752

hip-odoo/odoo,hip-odoo/odoo,bplancher/odoo,ygol/odoo,ygol/odoo,ygol/odoo,hip-odoo/odoo,ygol/odoo,ygol/odoo,bplancher/odoo,Elico-Corp/odoo_OCB,hip-odoo/odoo,hip-odoo/odoo,Elico-Corp/odoo_OCB,ygol/odoo,bplancher/odoo,Elico-Corp/odoo_OCB,bplancher/odoo,bplancher/odoo,Elico-Corp/odoo_OCB,hip-odoo/odoo,Elico-Corp/odoo_OCB,Elico-Corp/odoo_OCB,ygol/odoo,bplancher/odoo