ssh key management ui

Currently, we use GitHub to store user SSH keys. If a user doesn't have an SSH key, they'll have to add it via GitHub. We can detect this on signup and tell them what to do pointing to docs for now.

However, we should make it as painless as possible. So a step forward is to provide an SSH key manager interface that just wraps the GitHub SSH keys API. This way if they have no keys when a user signs up, they can add them right there in the signup flow. They can also manage them from the console later without going to GitHub.

This is also a step towards our own key management (#70).