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.)
If I get a validation error when I submit this form, it doesn't remember which mapping type I chose. We can make it do that with code like this:

```ruby
<% selected_type = @batch.type || 'redirect' %>
<%= radio_button_tag(:type, type, (type == selected_type), class: 'js-type') %>
```