Make MappingsController#find_global deal with sites

- Use a regular expression to check if a path is present in
  `url.request_uri`. If it's just a /, redirect to the site dashboard.
Rename the Go to Mapping modal to show that it works with sites
Remove inline onclick, replace with a module

* Avoid using the javascript onclick attribute
* Create a re-usable module that can be applied to any mousetrap key
combination and link

cc @issyl0
Tidy up jump to mapping modal and link

* Alter opening strap-line of app to be more helpful to new users, by
providing instructions rather than stats
* Keep the stats available in column headers
* Add a footer to the modal
Merge pull request #387 from alphagov/go_to_mapping_works_with_sites

Go to mapping works with sites