Missing mastodon link

[sertonix]

It would be nice to add a link to the mastodon account in _layouts/default.html too.

When done correctly the website will then be shown as verified: https://joinmastodon.org/verification

[dreua]

Would you accept a PR adding this? And should we also remove Twitte/X in the process? (I can't even view the content on X without an account. Very open platform and stuff 🤷 )