Create/edit subscriber modal shows device groups that do not exist

[patriciareinoso]

Describe the bug

Network slices contain references to device groups. The webconsole allows you to create a NS with a reference to X Device Group before actually creating the Device Group. In this case, the DG will appear in the list of possible device groups you can add a subscriber to.

The issue: if the DG does not actually exist, it should not be shown in the list of possible subscribers.

To create/edit a Subscriber we need to choose first, the NS and then the DG:

To think:

1. The DG should not be shown in the list if it does not actually exist. Should we filter better the list of DG shown?
2. OR to avoid this problem, should we first choose the DG from the list of exisiting DG? What happens if it is not associated to any NS? is it weird to attach the subscriber to a DG instead of to a NS?

Right now the DGs are really tight to NSs. We are planning to create a separate tab for DG (https://warthogs.atlassian.net/browse/TELCO-1404)
After that, idea 2. maybe would make more sense?

in the webconsole: If a Subscriber is added to a DG that does not belong to any NS, the subscriber tables will not be created because they need information from the NS.

Additional context

This is low priority, we can only reproduce this issue using the webconsole API. And after #716 the NMS will not crash if the DG does not exists .
My intention when opening this bug it to trigger the conversation about this subject.