pyqt5: make qtwebkit optional, disable by default

qtwebkit appears to be unsupported in Qt 5.11. We are using some old port
https://github.com/NixOS/nixpkgs/blob/475d653afdbd8fe3e00ccfd22a30014b0df7aeaa/pkgs/development/libraries/qt-5/5.11/default.nix#L39-L48
and it is broken on darwin.