Add redirect handler

Redirection behaviour is triggered by a `handler` value of `redirect`,
with the redirection target being set by `redirect_to`. An optional
`redirect_type` field can be added with a value of `temporary` to cause
a 302, otherwise it's 301 by default.

[#58080482]
Add `add_redirect_route` helper

This hides the implementation of redirection-type routes.
Explicitly skip prefix routes
Merge pull request #39 from alphagov/redirection-handler

Add redirect handler