Make the single edit mapping type labels clickable

- Before, only the radio buttons on the single edit mapping form were
  clickable, not the labels. This works by setting the `value:`
  attribute of the Rails `f.label` helper to whatever `type` is
  depending on the mapping, leading to greater consistency across the
  app.
Merge pull request #428 from alphagov/clickable_radio_button_labels

Make the single edit mapping type labels clickable