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.)
This and the one below should stay as `When` steps because they're actions, not assertions. Using `And` here implies that they are `Then` steps, which make assertions about the results of actions. [This post](http://xp123.com/articles/3a-arrange-act-assert/) explains the reasoning behind the 'Arrange-Act-Assert' pattern a bit more - Cucumber (or rather Gherkin) uses 'Given-When-Then' for this sequence.