* FIX: don't allow inviting more than `max_allowed_message_recipients` setting allows
* add specs for guardian
* user preferences for auto track shouldn't be applicable to PMs (it auto watches on visit)
Execlude PMs from "Automatically track topics I enter..." and "When I post in a topic, set that topic to..." user preferences
* groups take only 1 slot in PM
* just return if topic is a PM