Process uploaded CSVs in the background

This is necessary to support large CSV files (c. 80,000 rows).  Previously, these uploads were timing out, causing some strange behaviour in the load balancers resulting in duplicate services created.

This now temporarily stores the raw CSV data in the database, and has a delayed_job worker to process it in the background.  It also adds a uniqueness constraint to service slugs that would have prevented the duplicates being created.

I've also taken the opportunity to clean up some bits (e.g. it was using govuk_content_models for no good reason).