Implement `belongs to` in the popover

The `calendar-context` grasshopper branch contains everything we need to implement the `belongs to` field in the popover.

We now need to also provide the backend with some context when subscribing to a series by passing it `context=module.id` when subscribing.

![sadfasdfasdfasdfsdafdsfds](https://cloud.githubusercontent.com/assets/2194396/5468815/c2483aba-85cb-11e4-9482-80e53adc1498.png)
Needs `@private`