Remove SECRET_KEY default value from app config
Remove SECRET_KEY default value from app config

Missing SECERET_KEY env variable should always prevent the app from
working, otherwise errors in production configuration can mean the
app is silently using the default value, which is unsafe.

mtekel/digitalmarketplace-supplier-frontend,alphagov/digitalmarketplace-supplier-frontend,alphagov/digitalmarketplace-supplier-frontend,mtekel/digitalmarketplace-supplier-frontend,alphagov/digitalmarketplace-supplier-frontend,alphagov/digitalmarketplace-supplier-frontend,mtekel/digitalmarketplace-supplier-frontend,mtekel/digitalmarketplace-supplier-frontend