Add a supporting code to separate groups#show
and groups#details which is required for the
proper implementation of the Group Overview
content and Security Dashboard option for it

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.