Reverse statement proofs

[kostko]

We should specify a way to make reverse statement proofs of the linked accounts, for example:

• For URLs, a signed reverse statement to be published in HTML metadata or at a well-known path.
• For Twitter handles a signed reverse statement to be published as a Twitter post.
• For Keybase handles a signed reverse statement to be published in the Keybase filesystem.

The registry tooling would could then optionally check these reverse statements and reject invalid statements. The metadata registry could enable these checks and thus only accept verified statements (removing any which fail periodic re-verification).