Add new controller action and route

**WIP** - needs to be rebased onto #104 / #164 then refactored so that the search params aren't constructed in 2 different places.

This took me longer than I thought as with the controller overriding method I'm using, the new controller action isn't picked up, resulting in an `ActionController::UnknownAction` error. After much experimentation, the `ThemeControllerActions` module has gained a `method_missing` method to catch the missing controller action and pass it to the theme's controller instead. (Oddly, this second bit isn't strictly necessary as it appears to work by just rethrowing the error (presumably it slows it down for just long enough to notice the overridden method?!) but the longer version makes more sense.)