Server SSH fingerprint displayed in the Hub should support multiple formats (at least SHA256 but possibly others)

[JedMeister]

The Hub currently displays the SSH fingerprint as a hexed md5 hash. E.g.:

f6:fc:1c:03:17:5f:67:4f:1f:0b:50:5a:9f:f9:30:e5

As of OpenSSH 6.8, the default is now base64 SHA256. To support newer SSH clients, and the upcoming release of Debian Stretch, we should also support SHA256 (plus perhaps others).

[JedMeister]

Have unasigned @OnGle as we've decided to put this on hold for now. That is because it would not be fully backwards compatible.