Validate support for query strings in redirects

As part of some future campaign work we need to support redirecting vanity URLs to URLs that contain query strings.

The router API [currently doesn't support this](https://github.com/alphagov/router-api/blob/9cf4dd9f7de1331c2f5d76c6f23dfdb5bb88e7c9/app/models/route.rb#L86), but before I work on that I wanted to be explicit that the Router itself supports them.