LAST COMMIT ONLY -> pos part of event slots:

PURPOSE
=======

In this task, we add a new feature allowing events to be
'multi slots', which means an attendee can now chose a slot
in addition to a ticket. This commit adds the pos part to
this new feature.

CHANGES
=======

Now, when the event is multi slot, the first popup will
be the one for slot selection. Sold out slots will not
be selectable and will be disabled on that screen.

The availabilities for tickets will be computed based on
the selection made on that screen, and ticket popup will
be opened in turn. Those consider the various configurations
with slot and its seats_max, the tickets and their seats_max,
and the minimum between slot and sum of tickets availabilities.

- We include the new model event.slot and associated fields
in the loaded pos data.
- In order to support the new handling of availabilities,
the config popup is slightly modified. Also, we now use a
litteral 'unlimited' value for availability, which also
improves clarity as availability computation can be a bit heavy.
- A few display improvements are done in the product card.
This part was 'hidden' and is brought back at the same time.
- We only show future slots.
- Pos order lines will show the event and the slot. They will
be merged only if both are the same.

Task-4307566