Make unencoded slashes work when requesting a tag

Right now, using an unencoded slash character in a URL for a tag ID returns a 404 for the tag, because Sinatra can't correctly match the route. Using` %2F` works, but this gets clobbered by Nginx when you make a request through www.gov.uk.

This changes the route to use a wildcard match, handling the parameters as block arguments instead of using the `params` hash.