Use numeric ids for "supporting page" admin links

I've changed the url helper code to use numeric ids for the supporting pages in admin, and added a constraint to the route for the page to intentionally break old admin links which use the slug. This will stop this sort of URL being easily used in the future.

There's a destructive data migration here: I've checked SQL diffs by hand, so should be ok, but would be grateful for a review - thanks.