Migrate old supporting page records to editions

The old supporting pages table is left untouched.

Attachments have been updated (not copied) to point to the new
supporting page editions.
Handle supporting pages in document route helpers

Supporting pages are displayed on the front end in the context of a
policy, so the route requires a different path name and a policy_id to
work.

Also clean up the path helpers in Admin::SupportingPageHelpers.
Pending migration to remove supporting pages table