System groups should be treated like normal groups and display their list of nodes
In groups page, we have system groups. For them, we don't have the list of nodes, which is quite disturbing.
We should be able to see the list of nodes as for any other groups.
Can workaround the limitation with APIs, for example to see all nodes managed by relay with ID '717b63d1-01fe-4d4f-a7e5-cfb7c0d47b4f', one can uses:
curl -k -H "X-API-Token: the_token" -H "Content-Type: application/json" -X GET 'https://xxxxx/rudder/api/latest/groups/hasPolicyServer-717b63d1-01fe-4d4f-a7e5-cfb7c0d47b4f'
And with "jq" tools, one can even just get the list of managed nodes, one by line:
curl -k -H "X-API-Token: the_token" -H "Content-Type: application/json" -X GET 'https://xxxxx/rudder/api/latest/groups/hasPolicyServer-717b63d1-01fe-4d4f-a7e5-cfb7c0d47b4f' | jq '.data.groups.nodeIds'
But it became trickier for some special "magic" groups: "All Nodes", "All Managed Nodes", "All Node with a Server Role". These groups don't really hold any reference to nodeId, and so the user just can't see them.
So we need to 1/ return these groups in API, et 2/ let the user see content of all system groups.