Add Slack TFA monitoring

[radekk]

There is an API call which returns the list of users with information about TFA state. It should be rather straightforward to implement this monitor. Required scope is users:read

            },
            "is_admin": true,
            "is_owner": true,
            "has_2fa": false,
            "has_files": true

Ref: https://api.slack.com/methods/users.list