Delete artefacts when changed to nonindexed kinds

We have had some artefacts created (incorrectly) as Answers when they should
have been CompletedTransactions. These were not removed automatically. This
change would ensure that they are in future.
Small readability changes
Add tests for Search update observer
Remove bogus file
Merge pull request #57 from alphagov/remove_nonindexed_artefacts_when_changing_kind

Delete artefacts from search index when changed to nonindexed kinds