Allow Passphrase to generate SSH keypairs and extact public keys from private keys

Summary:
Ref T4587.

  - Add an option to generate a keypair.
  - Add an option to view the public keys for existing keypairs.

Test Plan:
  - Generated keypairs.
  - Viewed public keys.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: aran, epriestley

Maniphest Tasks: T4587

Differential Revision: https://secure.phabricator.com/D8515