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
Use a Set for better performance of whitelist
Remove unnecessary path munging

@rgarner pointed out that the app root is already on the load path.
Inline used-once filepath var
Merge pull request #13 from alphagov/use_whitelist_to_restrict_redirects

Use a whitelist to restrict redirects