Platform: Implement monitoring endpoints - GET subscriptions [#77728418]

TODO: describe your changes

===

Pivotal tracker story [#77728418](https://www.pivotaltracker.com/story/show/77728418) in project *Platform*:

> https://github.com/HouseTrip/routemaster/issues/12
> 
> **TEST PLAN**
> 
> set off a sync and check out 
> https://routemaster-staging.herokuapp.com/subscriptions
This is weird now... why not just 

```
get '/subscriptions' do
      content_type :json
      payload = Routemaster::Models::Subscription.map do |subscription|
        {
          subscriber: subscription.subscriber,
          callback: subscription.callback,
          topics: subscription.topics.map(&:name),
          events: {
            sent: subscription.all_topics_count,
            queued: subscription.queue.message_count,
            oldest: subscription.age_of_oldest_message
          }
        }
      end
      payload.to_json
    end
```