mastodon instance

I set up a mastodon instance for people to play with at w3c.social 
<https://w3c.social>. I'm playing with some integrations with aspects of 
W3C workflow, but nothing's ready quite yet.

Reminder: we could still use more AC responses on 
https://www.w3.org/2002/09/wbs/33280/socialweb-exnt/

Perhaps encourage them to try out w3c.social, although the value might 
not be obvious at this point.

       -- Sandro