Support JSONP callbacks for the API/v1 endpoints

SeaGL currently uses OSEM as a backend, but still has a static website
serving as our public interface. The API endpoints have a great amount
of data, but without the jsonp callback paramater, embedding the data
is a bit difficult. This change just adds the jsonp callback param
to all the API endpoints. This should be a NOOP for anyone using direct
json calls, but will allow cross domain requests that require jsonp.

app/controllers/api/v1/speakers_controller.rb

@@ -13,7 +13,7 @@ module Api
         end
 
         users = users.where(event_users: {event_role: :speaker}).uniq
-        render json: users, each_serializer: SpeakerSerializer
+        render json: users, each_serializer: SpeakerSerializer, callback: params['callback']
       end
     end
   end