Fix government association bug

Those with the relevant permissions can override a government
associated with a particular item of content - see
https://docs.publishing.service.gov.uk/repos/whitehall/history_mode.html#overrides.

The implementation was buggy insofar as Publishing API (and thus
Content Store, and the frontend) would always be one version
behind.

1. Document is associated with Government A.
2. You edit via the UI, to associate with Government B instead.
3. In the UI, the government shows as Government B, but the draft
   preview continues to show Government A. (This is the case
   even if you go on to publish the content).
4. You edit via the UI, to associate with Government C instead.
5. In the UI, the government shows as Government C, but the draft
   preview now shows Government B.

This appears to be down to unexpected caching within
`PublishingApi::PayloadBuilder::Links`, whereby calling
`item.government` returns the old government association. Calling
`item.reload.government` did not help, but fetching the government
ID and using that to instantiate a new `Government` object seemed
to work. This was established by adding a `byebug` within the
method and prying the `news_article_presenter` locally.

```
$ news_article.government
<Government id: 19, slug: "1935-to-1945-national-government", name: "1935 to 1945 National government", start_date: "1935-11-15", end_date: "1945-07-26", created_at: "2015-03-02 15:33:46.000000000 +0000", updated_at: "2019-11-25 11:05:27.000000000 +0000", content_id: "a05c2130-60c3-4d92-9f30-1cd3fb9d4a00">

$ news_article.government_id
25
```

This led me to narrow it down to the `@government` inline caching
in the Edition model. Fetchign the government 'fresh' on each
call eliminates this kind of bug.

Trello: https://trello.com/c/dGAmUBKc/2833-investigate-republishing-docs-reassociated-with-different-government

Stop caching `government` - see previous commits

app/models/edition.rb

@@ -348,9 +348,11 @@ def can_set_previously_published?
   end
 
   def government
-    return @government ||= Government.find(government_id) if government_id.present?
-
-    @government ||= Government.on_date(date_for_government) unless date_for_government.nil?
+    if government_id.present?
+      Government.find(government_id)
+    elsif date_for_government.present?
+      Government.on_date(date_for_government)
+    end
   end
 
   def search_government_name

test/unit/app/models/edition_test.rb

@@ -806,6 +806,18 @@ class EditionTest < ActiveSupport::TestCase
     assert_equal previous_government, edition.government
   end
 
+  test "instantiated edition fetches its most up to date #government association" do
+    current_government = create(:current_government)
+    previous_government = create(:previous_government)
+    edition = create(:edition, first_published_at: Time.zone.now, government_id: previous_government.id)
+    assert_equal previous_government, edition.government
+    # at this point, edition.government == previous_government
+    # But if the user updates the association to a different government, we don't want the old one to be returned
+    edition.government_id = current_government.id
+    edition.save
+    assert_equal current_government, edition.government
+  end
+
   test "#government returns the current government for a newly published edition" do
     government = create(:current_government)
     edition = create(:edition, first_published_at: Time.zone.now)