Report edition state counts to Statsd

This is mainly to help monitor problems with scheduled publishing (when the number of scheduled publications isn’t the same as the number in the Sidekiq queue), but it generalises quite easily to report on all the workflow states.

I also pulled out the existing counter-based reporting, partly because it wasn’t namespaced and partly because it wasn’t providing the numbers in a useful way.
Memoizing this won't have much effect as this instance variable won't persist between requests (A new controller instance is created for each request.)  Given `StateCountReporter` is inexpensive to create, it may be clearer not to memoize it at all.