Add a Cancel button to the bulk add preview page

- This uses the OffSiteRedirectChecker and params[:return_path] to
  return the user to where they were before they arrived on the bulk add
  page, so long as it was on the site, otherwise they get returned to the
  site mappings page.

(Thanks to @rgarner for helping with testing this.)
The other cancel buttons in the app aren't red, and they have a bit more space around them, [like this one](https://github.com/alphagov/transition/blob/master/app/views/import_batches/preview.html.erb#L41) on the import batch preview page:

![screen shot 2014-09-23 at 15 25 47](https://cloud.githubusercontent.com/assets/1822424/4373559/9043b408-432d-11e4-8bdf-cfcf26a7d0d1.png)

...whereas this one looks like this:

![screen shot 2014-09-23 at 15 27 50](https://cloud.githubusercontent.com/assets/1822424/4373609/ddfcab46-432d-11e4-96ce-3ca9d29d609b.png)