Leverage EntityOwnerTrait

Problem/Motivation

We have a number of entity types which implement EntityOwnerInterface: event series, event instances and registrants.

We are currently maintaining our own implementation of the methods that belong to this interface, but since Drupal core ships EntityOwnerTrait we can replace a bunch of code with the well tested core implementation.

When doing this we need to take care to retain the revisioning of the owner field, so we probably need to retain the code from EventUserTrait::ownerBaseFieldDefinitions(). See also the comment from @pwolanin at https://www.drupal.org/node/2998929#comment-14404239 →