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.
Not really, no. It was either hard code the current schema somewhere which would require the person making a schema change to remember to change it also, or just assume that we would be sensible with the dates and use the current date for the latest schema (the schema version is actually just a number).