Allow user to approve/discard changes after reviewing a diff of 'replaced' content

[tomusher]

When using a 'replace' prompt, it is often not clear what has changed,.

In pursuit of making wagtail-ai a tool to support editors rather than write content for them, we could consider:

• Showing a diff of changes that will be applied
• Allow the user to approve/discard/retry that suggested change

[thibaudcolas]

See #66