Blocks hooks

Obviously, I don't know how to rebase. :smirk: If you want me to close this and redo something I am happy to do so (with coaching).

This PR basically creates a hook for the Blocks module but only really implements the call for the Html block. This then allows the user to add Scribite to the HTML block for easier editing (@nmpetkov)

I will make another commit in a moment with a changelog entry.