Clarify that mappings aren't saved yet

* Rename submit mappings to continue
* Add a message which explains what will happen for the tentative user
Break redirect to URL on to new line

* Long target URLs are easier to read when broken away from the main
title and made a little smaller
On confirmation highlight existing paths

* @rjc123 suggested highlighting existing paths with the same colour
being used for the warning message at the top. I think this makes sense.