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.
add historic_appointments to application_helper

so chancellors, past prime ministers index and show pages highlight how gov works in the nav
Merge pull request #840 from alphagov/fix-nav-not-highlighting

add historic_appointments to application_helper
add urls to coming soon and exempt org pages

https://www.pivotaltracker.com/story/show/56771798
Merge pull request #830 from alphagov/remove-old-migrations

Remove migrations older than 30 days