Add GET request for API call to `/groups/{group-id}`
For groups we currently only have a GET on all groups and a POST on a specific group:
// Groups
groupsHandler := make(handlers.MethodHandler)
groupsHandler["GET"] = api.ListGroups()
r.Handle("/groups", groupsHandler)
groupHandler := make(handlers.MethodHandler)
groupHandler["POST"] = api.CreateGroup()
r.Handle("/groups/{group-id}", groupHandler)
(see api/v1/api.go)
Now it would be nice to be able to just check if a group already exists, with a GET on this group. Currently we have to fetch all groups and iterate through the resulting array to find out if a group exists.