settings: Disallow everyone group for new setting.

This is important because the "guests" value isn't one that we'd
expect anyone to pick intentionally, and in particular isn't an
available option for the similar/adjacent "email invitations" setting.

zerver/lib/types.py

@@ -287,6 +287,7 @@ class GroupPermissionSetting:
     allow_internet_group: bool
     allow_owners_group: bool
     allow_nobody_group: bool
+    allow_everyone_group: bool
     default_group_name: str
     id_field_name: str
     default_for_system_groups: Optional[str] = None