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?
This might not be an issue, but is there a chance that the `supporting_pages` will still contain the earlier, pre-updated slug here? Do we need to call `reload` on each page?