Change select boxes to radio buttons for mapping type selection

- Radio buttons make switching between types when editing a mapping
  easier. It's one click rather than 2/3, and you can see the options
  sooner. This has been a minor usability problem in user research.

(Thanks to @elliotcm for helping to get the JavaScript working with radio buttons.)
> I thought that the new form only deals with adding new mappings, there won't be any existing records?

You are correct - I didn't notice this was code for the `new` action, so it can be left out of this one.