[WIP] Separate controller action for the pins JSON

**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<a href="#footnote-1">[1]</a> 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.)

<a name="footnote-1"></a>
[1] Rails has moved the `UnknownAction` magic into the middleware layer which makes it impossible to trap in the application code
Since we know this is going to only be used for JSON, we can probably simplify the logic a little bit here and just do the full 1000 results search immediately. Then we also don't need to worry about passing the `page` or `authority` params into the search, since we know they won't be used. Nor do we need to set the instance variables like `@display` and `@map_display_possible`, the intention was that javascript would be calling this directly once it's loaded, so we don't have to worry about telling anything what to display.