Add DB schema version table

This page is a capture in the next bucket of the product backlog — a pre-sprint idea, not yet pulled into a sprint as a story.

At present we have no way of knowing what version of the schema we are running. We should probably look into sqitch.

Links:

• GH: golang-migrate: "Database migrations written in Go. Use as CLI or import as library."
• GH: tern: "Tern is a standalone migration tool for PostgreSQL. It includes traditional migrations as well as a separate optional workflow for managing database code such as functions and views."
• GH: sqitch: "Sqitch is a database change management application."