Add support for SFTP key authentication

Add support for external files accessed via SFTP using public key exchange authentication.
Keys are generated automatically when the configuration is added, or can be regenerated on demand if a key is compromised.
Creation of a new configuration row now triggers focus on that row. This is used to trigger auto-configuration for SFTP keys.
Generated public keys are saved in user's data directory for easy retrieval by an external application.
Add controller for SFTP key generation AJAX
SFTP class initialisation no longer produces a warning if the password field is missing.
Add unit tests for SFTP with key authentication backend