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?
The supporting pages don't actually contain their full slug in the database, they only have their part of the slug. They get their related policy and generate the full slug `:policy_slug/supporting-pages/:slug` when you call `search_index`. As far as I am aware reload just forces rails to reload the attributes on the object which are defined in the database. As the related policy isn't a column in the database calling reload won't actually have any effect. I could well be wrong there though.