Add 'political' property to hash

Content Publisher has two means of designating an edition to be
'political':

1. 'system political': if associated with role appointments or
   political organisations
2. 'editor political': if the publisher has explicitly marked the
   document as political via the "Gets history mode" checkbox in
   Content Publisher's UI.

For the purposes of importing these docs into Whitehall, we don't
particularly care the _means_ by which a document is considered
political. We don't need the overhead of having to map one 'flavour'
of political to Whitehall's representation of that 'flavour'. It
is enough to just pass the derived boolean from Content Publisher,
into Whitehall (which has its own 'political' boolean override
equivalent: https://github.com/alphagov/whitehall/blob/f5afce65514b02767df8625ccfa40c681046c95e/db/schema.rb#L444)

Author: ChrisBAshton

app/models/whitehall_migration/document_export.rb

@@ -23,6 +23,7 @@ def self.export_to_hash(document)
       summary: content_revision.summary,
       body: content_revision.contents["body"],
       tags: document.live_edition.revision.tags_revision.tags,
+      political: document.live_edition.political?,
     }
   end
 end

spec/models/whitehall_migration/document_export_spec.rb

@@ -102,8 +102,16 @@
       tags = { "primary_publishing_organisation" => [SecureRandom.uuid] }
       document = build(:document, :live)
       document.live_edition = create(:edition, :published, document:, tags:)
-      
+
       expect(described_class.export_to_hash(document)[:tags]).to eq(tags)
     end
+
+    it "has a `political` property" do
+      document = build(:document, :live)
+      document.live_edition = create(:edition, :published, document:)
+      allow(document.live_edition).to receive(:political?).and_return(true)
+
+      expect(described_class.export_to_hash(document)[:political]).to be(true)
+    end
   end
 end