Add a link to the Go to Mapping dialog from the homepage

- Only show this link if Javascript is enabled, as the dialog needs
  Javascript to work.
Merge pull request #386 from alphagov/add_a_link_to_go_to_mapping_dialog

Add a link to the Go to Mapping dialog from the homepage
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
Indicate on the site dashboard whether hosts are redirected by GDS
Make the redirector CNAME and IP into constants for clarity
Merge pull request #389 from alphagov/specify_hosts_redirected_by_gds

Indicate on the site dashboard whether hosts are redirected by GDS
Merge pull request #387 from alphagov/go_to_mapping_works_with_sites

Go to mapping works with sites
Improve jump to mapping copy without js

Currently, when JS is disabled the link disappears and the copy doesn’t
read correctly. Instead, hide the whole paragraph.
Merge pull request #390 from alphagov/fix-non-js-strapline

Improve jump to mapping copy without js