Re-introduce database migrations

We lost the ability to do schema changes when we removed EF (although in reality we could never do them easily). This re-introduces the ability using [Fluent Migrator](https://github.com/schambers/fluentmigrator/wiki).

When this gets pushed to `staging` and `production`, a manual database upgrade will need to be run to get the database into the correct initial state. The `ManualUpgrade.sql` is a script for this, but it might need tweaking depending on the actual names of the auto-generated key names.

Once the manual upgrade has run, further schema changes can be made by simply adding a new migration to the `DDDEastAnglia.DatabaseMigrations` project and it will get automatically applied next time the website starts.