PP data-in for service feedback: update url construction
Merge pull request #119 from alphagov/update-pp-data-in-api-url

PP data-in for service feedback: update url construction

* Currently performance platform provides two methods for writing to a data-set ('bucket'):
  - Writing directly into the collection name (which has underscores, because it's mongo)
  - Resolving which data-set is written to by providing `/data-group/data-type` e.g. 'apply-for-a-thing/customer-satisfaction'

* The second method is preferable in the case of customer satisfaction as it more clearly resolves to slugs from mainstream.