Change schools colleges policy slug

This is a rework of this pull request: #1893

- This should remove the policy and its supporting pages from the search index
- Update the policy slug
- Re-add the policy and its supporting pages to the search index
- Add a prefix redirect to the router to redirect both the policy and the supporting pages to the new location

Requested in: https://www.pivotaltracker.com/story/show/83664248

Do I also need add a matching redirect to router data?
As @edds says, the slug for the supporting doesn't include the policy slug, they are combined when the url/path is built. As the search indexing happens in a background job, we can rely on the latest version of the policy (with the new slug) being loaded at that point and the correct path being generated.