Merge "Notice" and "Callout" blocks

[ryelle]

The handbook code registers a "Callout" block, which behaves fairly similarly to the Notice block. It's confusing to have the two of them, so we should likely deprecate the Callout block (since the Notice block is styled to match the new redesign).

First, check if there are features of Callout to implement in Notice.

• [ ] Callout uses InnerBlocks and allows paragraphs & lists. Notice uses a RichText and only allows paragraphs & inline elements.