Remove migrations older than 30 days

We don't ever need to run these once all machines that need to apply
them to their database (i.e. production, preview, staging, CI, and
individual developer's boxes) have run them. There's therefore no reason
why we can't just clean them up every now and again.
Merge pull request #830 from alphagov/remove-old-migrations

Remove migrations older than 30 days