Migration 0051 adds ssh_keys.label. keys add --label <text>, defaulting to the authorized_keys comment; keys label <fingerprint> [<text>] renames or clears. Shown by keys list, admin user show, repo deploy-key list and the settings page. Deploy, runner and admin-created keys take the comment as their label; keys created at registration get no label, since RegisterAccount receives a parsed key with the comment already dropped.
Closes #208