Groups should not allow adding a member manually when a SCIM identity is deactivated

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Follow up from #208940 (closed).

When a SCIM identity is present for a given group and is deactivated (active is false) group maintainers/owners should not be allowed to re-add the user as a member.

Allowing the user to be re-added as a member when SCIM is deactivated should be fairly benign, since that doesn't necessarily mean the user can sign in to the group with SSO. However, as a matter of user experience we should display an error with an explanation.