Issue #2847651 by murrow, rachel_norfolk, mpdonadio, john@johncook.me.uk,...

Issue #2847651 by murrow, rachel_norfolk, mpdonadio, john@johncook.me.uk, BarisW, droplet, tstoeckler, andrewmacpherson, prabhu9484, ok_lyndsey, benjifisher, yoroy, lauriii, martin107: Improve timezones selector with optgroups