Add block slug as class for styling

[silveltman]

A quiet simple, yet useful feature: Add block slug (blockType) to the classes of that block. This would allow for custom styling based on which block.

This can be done either:

• Automatically
• Allow admin.className on a block

Some use cases for custom block styling:

• Hide buttons, for example delete button for required blocks
• Change appearance so that we can improve UX