Fine-Grained permissions

Currently we expose actions and properties to each action to allow users implementing custom permissions. We want to pass more data on each action to allow fine tuning permissions:

- The page being created/edited on Live Admin. For example pass the `page` data so one can define that only roles "admin" can publish HEEx pages or that the "external_contributor" role can only edit Markdown formats, and so on.

For now that's the only use case for this feature but it will define the API and documentation for future changes.