Add new resources via dropdown

Lots of links to create different types of resources were getting messy, so switched to a dropdown. Also wanted to refactor new and create controller actions to remove hardcoded references to classnames. These methods will now work even if class names change, or if resource types are added or removed.
Hey James, thanks for the awesome feedback! I was trying to be mindful of security in this refactor, but my thinking may have been flawed. I assumed that this could only create a new object in memory to be passed into the 'Add new resource' partial, but for it to be saved into the database, it'd need to pass through the valid_types check in #create_resource, and strong_params would also ensure its params were valid for the object type. Maybe you can explain to me where I went wrong later this evening!

I'm so glad to be learning about security before I'm actually working on business code... so thank you! Meanwhiles, I will take another pass at refactoring.