Figure out Group V1 vs Group V2

Signal-Android contains code for two versions of Signal groups. Groups V2 seem to leak less data to the Signal server.

Groups V2 seem to be enabled by OWS since half october: https://signal.org/blog/new-groups/

• Group links https://signal.org/blog/group-links/
• Newly created groups should be V2 groups
• What's Signal's migration strategy for V1 groups? See commit 985a220fc..5e536c3fa in Signal-Android.
• It seems that at least Signal Android warns the user that a V1 group gets created when inviting someone with WF ("an old Signal client"). When trying to add a V1-only client, this warning is replaced by a "they have to upgrade Signal", and the obvious inability to add a Whisperfish user.