foundation.mozilla.org: Add more styling options to `Callout` block

Description

Currently the Callout block only allows for bold text but more styling options would be helpful.

At the very least, it would be the best to add:

• Bullet list
• Links

Nice to have would be:

• Different heading styles

Screen cap of Callout block:

@nancyt1 can you please review this, is this what is needed for the ADG playbook?

Before adding ACs, I would like input from a dev on what styling options make sense to add from their perspective.

Acceptance criteria

• As an editor, I can add italic text to the callout block richtext
• As an editor, I can add links to the callout block richtext (using Wagtails link functionality similar to what is in Paragraph links - Internal link using page chooser, external link, Email link, Phone link, and Anchor link)
• As an editor, I can add numbered lists to the callout block richtext
• As an editor, I can add bulleted lists to the callout block richtext
• As an editor, I can add headings to the callout block richtext of levels H2 - H4
• As a page visitor, I see the callout block content render the same as other richtext on the site
• As an editor, I don’t loose any functionality that is currently on production (i.e. can still use Bold text)
• As a page visitor, I don’t loose any styling that is currently on production (i.e. grey background box and grid/width remain the same)