Change community domain name

[evanp]

I'm moving this issue from the w3c/activitypub#421 because I believe it is a data portability issue.

The primary use case is when when organizations set up a "test.example.com" Mastodon server, sign up a few dozen users, and use it for a month or two. When they decide to make it "production.example.com" or whatever real domain they want to use, they find out that all their previous content and connections are lost. Not good!

A similar use case is when the community domain name is retracted by the registrar. For example, the queer.af fediverse server run by ActivityPub co-author @erincandescent was disabled by the Taliban government which runs the Afghan top-level domain .af. This was resolved by the community through other means, but still represents an important test case.

A final use case is when the community willingly changes the name for some more benign reason -- a change of focus, etc.