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
Remove unused supporting page view
Remove supporting page block from fact check view
Tidy up supporting_pages#show view
Add published_ and active_supporting_pages methods

Also clean up the supporting pages controller to use the new methods
and the document routes helper for #index redirection, and update the
tests to ensure the correct path is used (slug not ID).
Remove force publish warnings for supporting pages

Now that supporting pages are their own thing we don't need to warn
about checking them before force publishing or scheduling a policy.
Remove supporting page code from search indexer

Now that supporting pages are proper editions they'll be indexed
independently when published without taking special steps when a
policy is reindexed or removed.
Add supporting page to admin new document dropdown

Also reorder the list to put the most commonly used types at the start.
Add context to supporting page preview dropdown

Show "View with policy" before links to make it clear what the options
are.
Rename related policies field for supporting pages

Be more explicit that it's a stronger relationship than related
policies normally are. Point out that multiple policies shouldn't be
common.
Add SupportingPageRedirect to handle slug changes

When converting supporting pages to editions some of the slugs had to
be updated. This model keeps details on which slugs should be
redirected and to where.
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.