Catalog of instances: on install, offer new instances a checkbox to register w/ a global index

[metasj]

Pitch

Make it easier to see a global index of instances (active and inactive), with basic metadata.

Add an option on installation for people who want to register in this way (w/o them having to choose where to register: the default a Schelling point that others can mirror).

Motivation

It's harder than it looks to let others know about a new instances.
It's harder than it should be to find out how to contact an instance owner and see basic stats on its activity (both while active and after becoming inactive)