Include the fingerprint of the securedrop submission key in the directory

[redshiftzero]

Suggested by @emkll: related to #400, if freedomofpress/securedrop#92 is ever implemented, we'd need to include the fingerprint of the securedrop in the directory so that sources have a place to cross-check they are encrypting to the right public key. We should probably do this regardless since some sources pre-encrypt in the current server-side approach.