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.
Yes, that is what it is for, although the ones that come out of the box don't give us anything that useful:

 - `NullAnnouncer` - does no logging
 - `ConsoleAnnouncer` - logs to `Console`
 - `TextWriterAnnouncer` - logs to a `TextWriter`

We could extend this (perhaps using `TextWriterAnnouncer`) to capture a log and put it in the admin section, but at the moment I don't think it worth the effort. From experience in developing this, if the schema upgrade fails you get an exception at application start up. Next time the application starts, you get the same exception as it re-attempts to upgrade the schema. This exception has been useful every time I got it.