[#60401480] Specs for redirect cache headers

All redirects should emit cache headers of 24hrs.
[#60401480] Cache headers for prefix redirects

Emitting `Expires` and `Cache-Control` as recommended by the RFC, to account
for HTTP/1.1 clients with HTTP/1.0 intermediaries.

Implemention for exact redirects to follow.
[#60401480] Cache headers for exact redirects

Cribbed `http.redirectHandler` from stdlib so that we can insert additional
headers onto `http.ResponseWriter` before calling `http.Redirect()`.

Moved the addition of header to a function, common to both, which modifies
the `ResponseWriter` in-place.
Merge pull request #63 from alphagov/60401480-redirect_cache_headers

[#60401480] 24hr cache headers on redirects