Remove old Statsd event counting.

This wasn't giving us useful metrics -- at least, it wasn't giving us
metrics we were using. Infrequent events like edition state changes
aren't ideally suited to the Statsd flush interval of 5 seconds.

Also, we've now settled on a namespace of `govuk.app.<appname>`, which
this predates.
Add a class to report state counts to Statsd.
Set up Statsd client directly in an initialiser.
Report edition state counts to Statsd.

These get reported in a filter, after any accounts that try to change
edition states. These reports fire even if the action fails, but since
they're just updating gauges, this shouldn't be a problem.
Report state counts after scheduled publishing.
Pull out shared code to set up state reporter.

Instead of having this declared twice in the Sidekiq worker and the
publication controller, let's set it up in an initialiser. We have to
set it up as a memoised accessor rather than assigning the value,
because it's hard to guarantee that the new initialiser will run after
the initialiser for the Statsd client. Memoising it means we can set up
test expectations without having to muck around with `any_instance`.
Merge pull request #220 from alphagov/statsd-state-counts

Report edition state counts to Statsd