Add a link to the side by side browser

* Link to the side by side browser for the given site
* Caveats: This isn’t a smart link and will show for sites without an
AKA domain, at the moment neither side by side browser or Transition
handles this gracefully.
Merge pull request #174 from alphagov/side-by-side-link

Add a link to the side by side browser on site dashboards