Add an EditionDeleter service

Also remove edition callbacks for :delete
Use EditionDeleter in admin editions controller
Remove unnecessary quotes in EditionUnpublisher
Stop using Edition#delete! in tests

Most of the tests just want a deleted edition to work with so we can
create one directly with a factory.

A couple of the tests just duplicate testing done in the EditionDeleter
test so have been removed completely.
Remove redundant edition workflow tests

The publish, force_publish and delete actions are all tested by the
service objects. Having these tests is redundant and could encourage
future developers to use the methods directly, which would be bad.
Merge pull request #1045 from alphagov/create-edition-deleter

Tracker: https://www.pivotaltracker.com/story/show/52741217

Create edition deleter