[Do Not Merge] Spike dual run content store on postgresql

[aldavidson]

Trello card, part of the overall Epic to migrate content-store to PostgreSQL

Adds two new applications:

• content-store-on-postgresql
• content-store-proxy - a simple sinatra app to sit in front of the MongoDB and PostgreSQL content stores, forwarding all requests to both

Also makes content-store-proxy respond to the hostname content-store.dev.gov.uk, and the original content-store respond to content-store-on-mongodb.dev.gov.uk, thus routing all requests to the content store from other local containers via the proxy.