Config endpoint

This MR replaces the legacy /configuration endpoint which has been moved to /v2.

The config endpoint provides the following data:

• authentication providers
• enabled features and their settings
• arbitrary UI-specific properties (passed through from config file)

Configuration file handling has been improved and now supports loading from multiple files:

• application.yml
• secrets.yml
• ui.yml

The configuration file has been restructured and now has the following main sections:

• system
• security
• features
• ui