Only initialise statsd in one place, and move description method

Sorry for the combined commit. The description method is only used in this
controller so let's declare it here.
Extract progressing editions into a new class

Most of the methods in the Admin::EditionsController were to do with
progressing editions and could more neatly be combined in a command
class. They probably need further refactoring and this gives us a
clearer base from which to do that.

Conflicts:
	app/controllers/admin/editions_controller.rb
Move tests for progress validation into unit tests

Now that I've extracted EditionProgressor our controller can rely
on that to do the right thing. In order to make sure it does do
the right thing it needs some tests. These tests are basically the
tests we already had for the old controller methods moved across.
Document a couple more arguments to EditionProgressor
Replace User object with dummy WorkflowActor

We don't need a full user object to test EditionProgressor, so let's not
have the overhead of creating one.
Remove residue of bad merge
Remove reference to class that doesn't exist in this branch

Sorry
Tidy up use of new_assignee method

Not sure if this was broken in a merge conflict or through the
cherry-picking used to break up this branch's predecessor. Tests
pass now.
Add missing newlines
Merge pull request #86 from alphagov/extract-edition-progress

Extract edition progress