Updated test description
Move worldwide priority/edition joins to document

Previously the worldwide priorities were joining to edition ids, which
is incorrect: if the document is subsequently republished, then we won't
be pointing to the right edition. This change appropriates the existing
join for policies and uses it for priorities also.

The migration is destructive (all existing links between priorities and
editions are removed), but because the code isn't live yet, this
shouldn't be an issue.

The related_documents method here actually is only needed in the tests,
and should probably be a property of the edition factories.
Fix regression caused by rename
Ensure that policies and priorties don't interfere

When associating policies and priorities to the same edition we
previously had clashes.

Turns out it's not actually possibly yet to associate policies
and worldwide priorities, but this may come up in the future and it's
good to have a central place that we record all edition -> document
relations that works as advertised rather than inconsistent behaviour.

At present the implementation of `related_policy_ids=` and
`worldwide_priority_ids=` isn't fully satisfactory: potentially this
could be refactored into the shared base class using a helper, rather
like `attachable`.
Test regression fixes

All caused due to renames
Merge pull request #326 from alphagov/edition-worldwide-priorities-relate-to-document

Edition worldwide priorities relate to document