Type AliasNotificationGroup

NotificationGroup:
    | MentionNotificationGroup
    | StatusNotificationGroup
    | ReblogNotificationGroup
    | FollowNotificationGroup
    | FollowRequestNotificationGroup
    | FavouriteNotificationGroup
    | PollNotificationGroup
    | UpdateNotificationGroup
    | AdminSignUpNotificationGroup
    | AdminReportNotificationGroup
    | SeveredRelationshipsNotificationGroup
    | ModerationWarningNotificationGroup

Group key identifying the grouped notifications. Should be treated as an opaque value.