Add ability to subscribe/broadcast to other streams over websockets not directly related to models

[C4ptainCrunch]

I'm adding a bit of context, since this comes from a Slack discussion.

One example of a non-model stream: let's say I want to use the django messages API, but without a redirect like a "toaster" popup sent from the server sometimes you might want a model, sure, but also I might want it sent from a signal, like when a friend logs into your app.