Add staging supplier id check

This refactors the supplier ID checking code to make it easier to add
more supplier IDs and environments.
Add denied supplier id to log
Merge branch 'add-staging-supplier-id-check' into add_new_supplier_ids_live
Add 6point6 supplier id to allowed
Remove 6point6 id as allowed and only add specific email address
Change to test email
Merge pull request #181 from alphagov/add_new_supplier_ids_live

Add new supplier ids live