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