update: Thunderbird logo

[redoomed1]

List of changes proposed in this PR:

• Update the company logo for Thunderbird, which was announced alongside the app's major UI refresh in version 115
• Replace external link to Arkenfox's GitHub repository with an internal link to its section on the Desktop Browsers page
• Add threat model label to Email Clients page
• Remove "(or MUA)" in the NeoMutt description since the acronym is not spelled out, and IMO, "Mail User Agent" is a less user-friendly term than "email reader" though both refer to the same thing: https://en.wikipedia.org/wiki/Email_client
• Make some other minor wording changes

[github-actions[bot]]

✅ Your preview is ready!

Name	Link
🔨 Latest commit	672953b03515ec32c3ea94158a208e7129b0821d
😎 Preview	https://2737--glowing-salamander-8d7127.netlify.app/

[privacyguides-bot]

This pull request has been mentioned on Privacy Guides. There might be relevant details there:

https://discuss.privacyguides.net/t/2024-09-08/20708/1