Bulk adding

This replaces the single new mapping form with one which handles multiple paths. Paths will all be archived or redirected to a single new_url. When this form is submitted the canonicalized paths are shown for confirmation. It also shows whether paths match existing mappings, letting the user choose whether or not to overwrite them.

There's a known caveat that paths mistakenly submitted that do not start with a '/' are canonicalized to an empty string and thus ignored. It might be wise to show warnings against these paths.