Allow redirects to external URLs or query strings

We should allow redirect targets to contain query strings so that we can
redirect vanity URLs to filtered pages or pages with tracking
parameters.

We should also allow redirect targets to be external URLs because
sometimes we need to redirect to the archive or to other government sites.
Constrain new validation to `exact` routes only

Given the uncertain behaviour of munging query strings/paths together
for prefix routes it's simpler to limit the new wider validation to just
`exact` redirects.
Merge pull request #55 from alphagov/allow-external-or-query-string-redirect-targets

Allow external or query string redirect targets