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