Fix frontend statistics announcement bug

The most recent change note should always be displayed regardless of whether it
was the most recent change or not.
Remove unnecessary indirection

The use of a separate presenter here on the statistics_announcements#show action
is unnecessary indirection as pretty much all the methods required in the view
are already present on the model. It also makes it more likely that bugs like
the one fixed by the previous commit will occur.
Merge pull request #1815 from alphagov/fix-frontend-statistics-announcements-bug

Fix frontend statistics announcements bug