Platform: Implement monitoring endpoints - GET topics [#74849038]

Pivotal tracker story [#74849038](https://www.pivotaltracker.com/story/show/74849038) in project *Platform*:

> https://github.com/HouseTrip/routemaster/issues/12
> 
> **TEST PLAN**
> https://routemaster-staging.herokuapp.com/topics (un: housetrip_avmczxf4dygleyjvudv6)
> and see cumulative counts for topics
We use extensively `let` - but it doesn't prevent to use `before` either. You can have `before { perform }` then. There were some discussion about this one in particular (`perform`) - and it is actually quite nice to have the perform directly in the test, it makes it easier to change. As far as I know, both are still used and are fine.