Design options bar for blocks

How do we do the options bar for blocks?

Open questions:

- How do we handle common options? Spoilers? Code blocks (make sure you can get out of them, unlike in the old editor)?
- Some discussion was had about possibly only having a single embed button, and having the editor automatically detect the source. In that case, do we really need a separate floating menu and block menu, or should we consider a single, always available, toolbar?