Require opt-in for Stellar reverse account lookup

When I use my default Keybase Stellar wallet to pay in any store outside of Keybase that accepts Stellar, my account id will be logged. This is no big deal, it's how Stellar works.

But this also allows the store or anyone on the internet to identify my Keybase username with all the proofs that this brings, even if they didn't know my Keybase username from the start.
Imagine the profiling and tracking you could do in the real world for advertisements etc.

The Keybase federation server makes this possible by allowing reverse account lookups. I think this should only be enabled for an account if you actively opt in for it.

Start by locating the federation server's reverse account lookup path and reviewing how account opt-in state could be represented. Done means reverse lookup is unavailable for accounts unless the owner actively enables it, while normal Stellar payments continue to work.

Written by the indexing model from the issue text.