Use a whitelist to restrict redirects

We have many mappings that are to government-held domains but which do not end
with .gov.uk. We also have mappings that are to general domains, such as
youtube.com.

The whitelist was copied from the redirector app:
https://github.com/alphagov/redirector/blob/master/data/whitelist.txt

If @psd and @rgarner could both cast their eyes over this, that would be great.