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
Could `map` be used here? Like in

```ruby
ret = Routemaster::Models::Subscription.map do |subscription|
  {
    # ...
  }
end
```