Purpose of this commit is to try to lessen random conditions being concatenated
in templates by correctly computing event_registrations_open field that is
now correctly based on
* event.date_end -> if event is done, registrations are not open anymore;
* event.start_sale_date -> lowest start date of tickets (if any; start_sale_date
is False if no ticket are defined, see _compute_start_sale_date);
* any ticket is available for sale (seats available) if any;
* seats are unlimited or seats are available;
Some better timezone computation is included even if it could be done better.
Task ID 2228189
Community PR odoo/odoo#48652
X-original-commit: a11af9074499465b6dd8ad1400e60c7af96e21c7