Because we ignored the format, a request to `/groups/foo/labels.json` would
redirect to `/groups/foo/-/labels/.json`. But really, it's worse than that,
because unless the request contained a trailing slash, we shouldn't add one.
Now, we only _keep_ a trailing slash, but don't _add_ one.
Rails' "url_helpers" method creates an anonymous Module (which a bunch
of methods) on every call. By caching the output of this method in a
dedicated method we can shave off about 10 seconds of loading time for
an issue with around 200 comments.
