Long description

[mattwynne]

There's a field called "long description" in the admin screens which doesn't show up on our site, but shows up on mastodon server discovery tools, e.g. https://mastodon.help/instances?lang=112&desc=nelson.social&minu=10&maxu=30000&minau=10&minki=500&noxious=1&creg=1&appr=0&lcok=0&ord=rand&advc=0&p=1

We should pull some stuff from the About this server text.