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. New databases should be setup with the `db:schema:load` rake task instead. There's therefore no reason why we can't just clean them up every now and again.

I'm not expecting somebody to just merge this – we should probably chat about the pros and cons amongst ourselves first.