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

TODO: describe your changes

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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
It's not really possible to do this in the database:

The database is Redis, these are very thin POROs, and even when we scale things up it'll me at most 10s of entries.
I think we're safe enough?