Add a "Generate Keypair" option on the SSH Keys panel

Summary: Ref T4587. Add an option to automatically generate a keypair, associate the public key, and save the private key.

Test Plan: Generated some keypairs. Hit error conditions, etc.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: aran, epriestley

Maniphest Tasks: T4587

Differential Revision: https://secure.phabricator.com/D8513