Change response format for subject of event-counts endpoint results

In an attempt to make the data structure of the response format
for the event-counts endpoint a bit more consistent, we've
decided to introduce the keys `subject` and `subject-type` at
the top level of each result map, rather than the variable
list of keys such as `resource-title`, `resource-type`, `node`,
etc.
Fixes based on pull request feedback
Use threading operator to improve readability
Merge pull request #644 from cprice404/feature/master/PE-1473-add-subject-key-to-event-counts-response

Change response format for subject of event-counts endpoint results