Don't broadcast messages to the social api when we don't need to, to avoid console warnings

We're getting lots of console warnings along the lines of:

```
[10:40:29.374] WorkerAPI: topic doesn't have a handler: 'talkilla.login-success' @ resource://gre/modules/WorkerAPI.jsm:41
```

This is because we're broadcasting all our `talkilla.*` messages from the worker to the browserPort (aka social api port) as well as the other ports. We shouldn't need to do this, and cleaning this up should make it clearer when looking for issues via the browser console.

Suggest review from @fqueze