Implement support for creating advanced blocks (create + preview)

[bradenmacdonald]

As an author, I can add any enabled advanced component to my content library using "Add Content" > "Advanced":

And then I can see a preview of them on the "Preview" tab:

They appear with grey cards, as seen above.

[ChrisChV]

@bradenmacdonald I plan on using an environment variable to control which advanced blocks can be created. This will be for now, until this configuration screen is in place. Is this ok?