Add online cargo to check if user is online/offline

[tomaszantas]

The frontend uses check_internet_connection command to check whether the app is online or offline.
Now, the command always returns true. It can be fixed with online crate

Motivation:

This feature has been moved partially to the backend, because the frontend (Javascript) do this job partially.
The Javascript's online listener detects only if the machine is connected to the router. So if it's not connected to the router, it returns offline. But if it's connected to the router, but router is not connected to the Internet, it also returns it as online.