* add conversion rate to Stats API timeseries

* make sure CR can be queried as the only metric

* add a test asserting zeros are returned

* add tests for filtering by other properties at the same time

* Remove unnecessary validation of params

1. It doesn't make to validate `interval` (and its granularity) in all
   endpoints. It's only relevant for the main graph.

2. The plug (renamed to `date_validation_plug`) already makes sure that
   the dates are validated. No need to call the same function again in
   Top Stats and Funnel endpoints.

* add metric validation to main graph

* Add tests for main graph API

* put conversion rate on the graph

* update changelog

* Add revenue metrics into metrics.ex

* make fn private

* avoid setting graph metric to visitors in goal-filtered view