Merge branch 'dz-refactor-group-routes' into 'master'

Refactor group routing

## What does this MR do?

Refactor group routing: 
* separate controller actions from nested resources
* prepare group routing for nested namespaces support

## Why was this MR needed?

So when we introduce nested groups support we need to only change `:id` to `*id`

## What are the relevant issue numbers?

Extracted from https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/7121

See merge request !7328