Fix the version table pushdown issue introduced with the example URL

The example URLs are typically long enough to run into the Versions table.
This means they get pushed under the form. If we put the example URL
in its own block, this doesn't happen.
Help list of mappings on narrow screens

(by putting redirect new_urls on a separate line with a
 subtle 'redirects to' span)
Mappings for 301s can no longer be created with no New URL