Allow redirect to *.gov.uk and simplify whitelist

This should greatly reduce any future maintenance on the whitelist, which is already out of date from the source in redirector. Several domains have been added: https://github.com/alphagov/redirector/commit/9f254cc6744d18f087e65aa40a30825262d9abc9

Need to check with @psd to see if there is a reason this hasn't been done already.