Enable MappingsController#find_global to find a site without a scheme

- Now 'foo.com/bar' works, not just 'http[s]://foo.com/bar', in the "Go
  to Mapping" dialog and other places where
  MappingsController#find_global is called.
Merge pull request #383 from alphagov/go_to_mapping_can_work_without_scheme

Enable `MappingsController#find_global` to find a site without a scheme
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
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.