Media must be referenced by UUID, not ID

Problem/Motivation

With Default Content API in core one can now easily import content with a Recipe. This module doesn't integrate well with this workflow as it references media (image/video) by ID, not UUID. The ID of the content in the destination environment can be different than ID in the source environment but UUID remains unchanged hence it makes sense to use the latter.

Steps to reproduce

Export content using default_content module and import with recipe using new Default Contet API in core.

Proposed resolution

Change from using ID to UUID when storing content. This way the reference will be maintained even if content is copied/moved between environments.

Remaining tasks

User interface changes

API changes

Data model changes