SSH Keys - allow viewer to download the private key

Summary: ...cuz otherwise this is pretty pointless. Fixes T5931.

Test Plan: generated a key, downloaded the key. previously download failed.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T5931

Differential Revision: https://secure.phabricator.com/D10311