Only use realtime when a realtime url is defined

Currently, we always try to connect to a realtime server.

This is not adapted to several use cases:

• offline
• local edition with the file system

Proposed fix:

1. only try to connect to realtime if a realtime url is defined
2. only set realtime url on the web example config
3. introduce an autosave when realtime is not activated
4. hide the realtime specific UI elements when not activated