Make Frontend::StatisticsAnnouncementFilter less clever

The Frontend::StatisticsAnnouncementFilter was written to accept both slugs and
actual Topics/Organisations for reasons that are unclear. This opens it up to
potential shenanigans as the dynamic find_by_ methods appear to behave strangely
when handed a hash.
Merge pull request #1797 from alphagov/fixes-for-stats-announcements-filter

Make Frontend::StatisticsAnnouncementFilter less clever