[MOV] (website_)event(_sale/_question/_track): split main model files

PURPOSE

As event will soon evolve (onchange -> compute, code improvements, addition of
new features) cleaning and improving tests is necessary to help avoid issues.

SPECIFICATIONS

As event model grow in complexity and features, it is easier to find its
way through the application with having registration model lying in its
own file to separate it from event-specific models (event.type, event.event).

Ticket (event_sale) and sponsor (website_event_track) models are also extracted
in their own file.

LINKS

LINKS

Side effect of Task ID 2089156 (event onchange to compute)
Community PR odoo/odoo#43127
Enterprise PR odoo/enterprise#7656