Filter stats announcements in admin by title/slug

www.agileplannerapp.com/boards/173808/cards/5170

this is just a first-pass at getting a search and
filter interface in place for stats announcements.
editors have an immediate need to start searching
by title and slug, so we put in place that first.
Merge pull request #1622 from alphagov/search-statistics-announcements

Filter stats announcements in admin by title/slug