Undo – Redo: existing reusable blocks create an undo level

[afercia]

Tested on latest master 4.5.1

• open an existing post that contains reusable blocks
• the reusable blocks take a few moments to fetch content and render, this is expected
• when they're fully rendered, the Undo button is active indicating there's an undo step in the history:

• click the Undo button
• the reusable blocks content disappears and a message "Block has been deleted or is unavailable" is shown:

• click the Redo button: the reusable blocks content gets restored

Expected: the Undo step to not be created and the reusable blocks content to not disappear

[aduth]

Related: #8119