Usability improvements for bulk adding

* Highlight existing paths on the confirmation page
* Update header to match mapping index view
* Remove some spacing between fields in the form for consistency
* Add messaging to clarify that mappings won't be saved immediately and to explain a bit about what happens
  * Make the button text say 'continue' for the same reasons
* Split the redirect-to URL onto a new line for better readability
* Make it clear that full URLs can be entered into the old URLs field

Many of these improvements came about after pairing with @rjc123.