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.
- Make the radio button style consistent with the non-JS filter mappings
  radio buttons: all on one line.
Fix indentation of the Type label and radio buttons in forms
Use the existing mapping type as the selected radio button on edit

- Switch to using the Rails form helpers in one case.
Merge pull request #411 from alphagov/change_select_to_radio

Change select boxes to radio buttons for mapping type selection