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.
A couple things here. 
- #p is already a ruby kernel method. Probably don't want to override that here.
- One-letter variable names are really confusing for people looking at them a few months down the line. This may be a trivial case, but it's a bad habit to get into.
- Probably no need to have this in its own variable as it's not modified, or actually used beyond the next line.
- The case statement was more secure as it defined all legal values of params[:type]. You could come up with other ways to be just as secure, but this isn't one of them. There are quite a few values a hacker could pass in params[:type] which might give away the internals of the server etc.
- Perhaps this logic could be pushed down to a private method, service object, or even a class method on Resource.
- We will start having to deal with these exceptions when they're raised. Otherwise people will just start getting server errors (500)