feat: add mastodon and whatsapp to social link icons

[hspiess-correctiv]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

The PR adds support for Mastodon and WhatsApp in social icons blocks for the Newsletters editor.

Once it is merged, the list of supported social icons will need to be updated here.

How to test the changes in this Pull Request:

1. Apply this PR and run npm run build.

2. Create a newsletter and add the Social Icons block (or start from a template that already has it).

3. Add the Mastodon and WhatsApp icons; confirm they're available.
   

4. Send a test newsletter; observe the Mastodon and WhatsApp icons are rendered as expected

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[chickenn00dle]

Just noting I'm adding the design review label @thomasguillot, to confirm the logo and colors are fine.

[thomasguillot]

LGTM. Thank you for your contribution @hspiess-correctiv

[github-actions]

🎉 This PR is included in version 3.28.0-alpha.1 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀

[github-actions]

🎉 This PR is included in version 3.28.0-alpha.1 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀

[github-actions]

🎉 This PR is included in version 3.28.0 🎉

The release is available on GitHub release

Your semantic-release bot 📦🚀