Link to the side-by-side browser for each mapping

- If the site has an AKA domain configured and the user has
  permission to edit mappings for the particular site.
- Make the edit mapping button slightly bigger as per @fofr's
  prototype.
Refactor the aka_host checking lines into a helper method
Check that the site isn't live before showing preview links

- If the site is live, the side-by-side browser is no longer needed.
Add failing Cucumber features for the new Preview button behaviour
Change a negation of .any? to .none
Make the side-by-side browser URL generation into a helper method
Fix the positive test for if the preview link displays

- Make the negative test preamble create a mapping just so it doesn't
  pass just because there are no mappings listed for the button to
  appear against.
Merge pull request #266 from alphagov/add_side_by_side_browser_link_for_each_mapping

Add side by side browser link for each mapping