Public or private dashboard visibility

You can now choose whether your Hive instance requires login. Set HIVE_VISIBILITY to private to gate the dashboard behind authentication, or leave it as public to let anyone use the instance without signing in. The login page remains available in public mode so admins can still sign in. This is controlled by a new RequireAuthenticated plug that redirects unauthenticated visitors to /login when the instance is private. (https://github.com/tuist/hive/pull/1, https://raw.githubusercontent.com/tuist/hive/server@0.2.0/lib/hive/auth.ex, https://raw.githubusercontent.com/tuist/hive/server@0.2.0/lib/hive_web/plugs/require_authenticated.ex, https://raw.githubusercontent.com/tuist/hive/server@0.2.0/README.md)