Only allow redirects to *.gov.uk

Whilst we should be preventing mappings to anything but *.gov.uk at the input
stage, we should provide an additional protection against gaps in that defence
and against the possibility of the database being compromised.
Only allow homepage redirects to *.gov.uk

Whilst we should be preventing mappings to anything but *.gov.uk at the input
stage, we should provide an additional protection against gaps in that defence
and against the possibility of the database being compromised.
DRY it up a little
Merge pull request #12 from alphagov/only_redirect_to_govuk_domains

Only redirect to *.gov.uk domains