Spaces: Enable sharing with groups

[tbsbdr]

Steps to reproduce

1. Login as user with the role "Space Manager"
2. Try to share a whole Space with a group
3. Sharing a Space with a group is not possible

Expected behaviour

• Sharing with a group should be possible

Actual behaviour

• Sharing a Space with a group is not possible

[kulmann]

current state: not possible backend side. I suppose allowing groups as members creates too much potential for conflicts in deduplication and auto-guessing the resulting membership role.

[tbsbdr]

CC @hodyroff 👆

[JammingBen]

This has been tackled via #8161.