Better entity usage integration in gutenberg

[chindris]

There is currently an entity usage integration in the silverback_gutenberg module that tracks the usage of embedded media. However, we can have cases when the content gets embedded into the editor in different ways: links paragraphs or custom blocks which make use of the link control (or the url control). We should also track those.

A proposal would be to track every block which has the following attributes: entityType and uuid.

To be on a safe side, the uuid could either be a string or an array.