* add metric validation + support in aggregate

* add a test ensuring comparison works

* disallow time_on_page with a goal filter

* Return time_on_page as `nil` from aggregate API

In case time_on_page cannot be calculated, we'll return it as `nil` from
the Stats API.

This is to make the behaviour consistent between breakdown and aggregate
endpoints. As for the UI, we'll still continue to report time_on_page as
0 - not changing any UI behaviour as discussed with Marko.

* add tests for time_on_page in event:page breakdown

* update changelog

* invalidate time_on_page with event:name filter

* add the ability to only query time_on_page in page breakdown

We'll need the visitors metric to get the list of pages to calculate the
time_on_page for.