[REF] calendar,web: update to FullCalendar v4

This commit updates the FullCalendar library from v3 to v4.

The purpose of this update for us is to add a proper support of the
drag-and-drop of events on mobile.

But also to fix scrolling issues present mainly on smaller screens. In
those configurations multi/inner-scrolls were present resulting into
either the inability to properly scroll the desired content or even
blocking alltogether the scroll.

As a nice-to-have feature, the version 4 removes its dependency on the
jQuery library, allowing a faster loading of a bigger set of events.

Last but not least, this version also reduces its dependency on the
Moment.js library (but still keeps a binding to it for compatibility
purpose).

This commit focus only on the update of the library itself and the
adaptation of the related code to keep it working. Further
enhancements/fixes will be done later on.

To do so it modifies the following points:
* Update the library in itself (js, css...).
* Convert all FullCalendar's method calls to the new syntax (e.g.
  `$calendar.fullCalendar('destroy')` becomes `calendar.destroy()`).
* Add a utility method to convert v4 events into the old v3 event
  structure to ease the transition from one version to the other.
* Wrap all event's custom fields into a new event's
  `extendedProps` object (as its the new way of storing them instead of
  directly as a prop. of the event).
* Adapt `slotLabelFormat` and `weekNumberCalculation`.
* Use the view's new names:
** `agendaDay`  -> `timeGridDay`
** `agendaWeek` -> `timeGridWeek`
** `month`      -> `dayGridMonth`
* Use the option's new names:
** `viewRender`      -> `datesRender`
** `eventMouseover`  -> `eventMouseEnter`
** `eventMouseout`   -> `eventMouseLeave`
** `isRTL`           -> `dir`
** `selectHelper`    -> `selectMirror`
** `weekNumberTitle` -> `weekLabel`

Ref:
https://fullcalendar.io/docs/upgrading-from-v3

Task ID: 2092550