Usability improvements for bulk adding

[fofr]

• 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.

[jamiecobbett]

I like this 👍