Content Translation does not use EntityOwnerInterface properly

Problem/Motivation

If you have an entity type that implements EntityOwnerInterface but does not use uid for the owner field, then content translation will add a content_translation_uid field instead of using the existing owner field.

Proposed resolution

1. Remove the $this->checkFieldStorageDefinitionTranslatability('uid') check in ContentTranslationHandler::hasAuthor()
2. Use EntityOwnerInterface::setOwner() in ContentTranslationMetadataWrapper::setAuthor() if there is no content_translation_uid field

Remaining tasks

User interface changes

API changes

Data model changes