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
Really restrict viewing of the leaderboard to only GDS Editors

- Currently, anyone with any permissions guessing the `/leaderboard`
  route can view the leaderboard as the GDS editor permission check only
  hides the menu option. Hide the page as a whole for those people not
  GDS Editors.
Merge pull request #391 from alphagov/restrict_leaderboard_to_gds_editors

Really restrict viewing of the leaderboard to only GDS Editors
Move Bulk Add into a separate controller

First step in cleaning this code up.
Simplify BulkAddBatchesController#create_multiple

Now that we have a dedicated controller, we can make this route a member route
instead of having it as a collection route and faking the member-ness.
BulkAddBatchesController#new_multiple => #new

This makes it more Rails-y and more consistent wth ImportBatchesController.
BulkAddBatchesController#create_multiple => #import

"create_multiple" no longer makes sense. The resource this controller manages
has already been created. The natural thing to call this method is "process"
but that can't be used because Rails uses it internally.
BulkAddBatchesController#new_multiple_confirmation

Rename this to be more Rails-y. This method creates the resource this controller
manages, so create is the right route to use.
Add BulkAddBatchesController#preview

No one likes rendering a page on POST. Redirecting to a URL is nicer because:
* you can reload
* you can navigate forward and back to the page without being prompted to
  resubmit the form

It also brings us into line with ImportBatchesController.
Make before_filters simpler and more consistent

Make it more like ImportBatchesController.