Allow claiming of no account on supported sites

If we do not have an account on reddit/HN/twitter/etc, we should to be able to have our profile publicly state this.

Obviously we cannot prove we don't have an account, but if someone trusts our identity on keybase it would be nice for them to have an 'official' statement of no presence.

The issue proposes allowing a Keybase profile to publicly state that the user has no account on supported sites such as Reddit, HN, or Twitter. Start by defining how supported sites and these claims would be represented, what trust or verification limits apply, and the acceptance criteria for publishing and displaying the statement.

Written by the indexing model from the issue text.