re-fetch group information

[sehaas]

The bot cannot reply to groups if it was added after startup, since the receiver lookup fails.

Instead of periodically refreshing the groups, the commit checks the group while receiving new messages.
If the group is unknown, _detect_groups() is called before passing the message on.

[Era-Dorta]

Thanks for the fix. The code changes look good but the unit testing is failing, I guess because of how the connection is mocked. Could please fix that?

[Era-Dorta]

Great. Thanks for fixing the unit testing too.