[IMP] event{,_sale}: unstore seat fields

With stored computed seat attributes, the database can be flooded with update
queries for the stored values for the event (ticket) seats computations (such
as reserved, expected, and available seats).
This can especially occur when a communication is sent to many people about
an event with a registration link, many users may want to register at the same
time, possibly resulting in concurrent_update errors.

In this commit, we remove the `store=True` attribute of those fields, and
therefore remove the Reporting/Event feature depending on them and rewrite some
domain searches and _compute fields in the event and event_sale modules.

This also impacts the way constraints are enforced on the number of
registrations vs defined maximum as no stored value is directly available.

For performance reasons, all events and tickets are now shown on backend form
views, with seat availability added in their displayed name.

Misc
To avoid delaying the inevitable, the Event configurator modal/wizard now
validates event/ticket consistency at closing.

The UI of the RegistrationEditor wizard is also improved:
* A warning alert will tell users that free registrations were not confirmed
because of insufficient seat availability.
* A first step to better explain the consequences of the actions taken on the
modal was to be taken, here via the description and buttons wording.

Tests
Query counts are (indeed reduced) and updated. However, as local testing with
`test-tags=/test_event_full` ("tef_only") is currently not reliable, these
values were updated by applying the same change from the commit as the one seen
for the runbots, while a "?" is appended to show this uncertainty.

Task-2654816
See odoo/upgrade#3118

Part-of: odoo/odoo#81583