[REF][MOV] website_event(_sale): support tickets in website_event frontend

PURPOSE

Integration between event and eCommerce is required only when users handle the
entire selling process online. However users may require ticketing support
while managing payments outside of Odoo. Purpose of this commit is to support
tickets directly in event application without need of sales.

RATIONALE

Remove the need to have event_sale installed to manage basic multi ticket
event type. Integration with eCommerce is needed only when one wants to handle
the entire selling flow online, i.e. order, payment, ... Integration with
Sales is needed only when one wants to create sale orders linked to attendees.

Many event users do not need all of this. Their attendees pay through bank
transfers or they simply manage payments outside of Odoo while still
requiring tickets management.

SPECIFICATIONS

In this commit we move frontend part of ticket support from website_event_sale
to website_event. Now eCommerce / event integration adds only payment
information when registering.

About tickets

  * if there is no ticket -> generic registration allowed;
  * if there is one ticket -> quick registration box;
  * more than one ticket -> unfolding registration box with all available
    tickets;

About price

  * sale not installed -> no mention of price. A ticket without price is not
    free. Its description allow to tell how to pay for example;
  * a price is set: price is displayed;
  * no price is set: FREE is displayed;

Most event frontend templates and controllers are therefore moved from
website_event_sale to website_event. Only part about pricing and sale order
creation is now located in website_event_sale.

Buy flow remains mainly untouched. Indeed this commit is mainly about moving
template to support tickets.

LINKS

Task ID 2177281
Community PR #43488