Put access limiting back onto Edition#organisations
Move eager publication/announcement loading to classes.
Fix fact check controller test.

Policies no longer show supporting pages for fact
checking.
Remove non-Rails code from AuthorityTestHelper
Fix validation approach for SupportingPage#related_policies
Overhaul admin/supporting_pages_controller test

Now that supporting pages are editions we are just inheriting shared
edition controller behaviour so we only need a very reduced set of
tests.
Move AlternativeFormatProvider from Policy to SupportingPage
Clean up supporting page unit tests. Lots of skips.
Whitespace.
Bin factory test.
Test govspeak with real supporting page URLs.
Update supporting page controller tests
Remove special code for doc mapping supporting pages.
More assigning supporting pages correctly in tests.
Route to new SP page from policy regardless of state.
Supporting pages can only exist on policies.
Supporting pages are no longer copied.
Use correct factories for testing SP attachment access.
Remove SP-specific routing tests.

This is now handled as part of being an edition.
Remove obsolete Edition::SupportingPages tests

Supporting pages aren't destroyed or copied when modifying the edition
they support.