so there's `user.challenges`, which appears to be being managed on leaving/joining challenges (don't know if we're otherwise using it in any way though). We could do

```javacript
if (!_.isEmpty(_.intersection(_.pluck(group.challenges,'id'),user.challenges)))
 ...
```

Do you think it's worthwhile? Or do think you think maybe we should just update the popover wording something like "if you haven't subscribed to any challenges, click anything"