Handle old admin supporting page paths in govspeak

Supporting pages are now editions so have admin paths of the form
/admin/supporting-pages/:id, but some existing documents will contain
links of the form /admin/editions/:policy_id/supporting-pages/:slug.

Note: supporting pages which have had their slug changed as part of the
supporting page migration will be incorrectly matched by this so the
links will be wrong.
Remove supporting page code from scrub-database test

Supporting pages are now just editions so future content will be
anonymised by the edition scrubbing.

The scrubbing of the old supporting pages table needs to continue to
take place until the table is dropped from production.
Add EditionedSupportingPageMapping

Keep a record of old supporting page IDs and their new ID in the
editions table. This lets us rewrite old style admin URLs relatively
painlessly.
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.