System actor to subscribe to ActivityPub based relays
ActivityPub relays work slightly differently than the Diaspora one. They're really just bots that will share posts to anyone who follows them. This creates a problem that if one follows a relay, you end up with all kinds of crap in your stream.
To support AP relays, we need to work around this fact with a system actor that can be configured to pull a list of relays from the-federation info for example, subscribe to them and thus pull in content to the system, without needing real users to do that.
Setting should default to be inactive.