Add a Custom HubL module to the new design manager

[elmo]

There's still no Custom Hubl (or Custom HTML, for that matter) module in the new template designer. There's still the possibility to switch back to the old designer, but it's being phased out in less than two months. After that, it becomes impossible to add Custom HubL modules to templates done with the drag & drop design.

Yes, there are the custom modules, and I'd be glad to use those, but they don't support including Global Groups with HubL. I'm already doing a design that depends on the ability to conditionally include Global Groups, so this is quite critical for me. Also, I'm using this inside another Global Group, so I can't move the conditional logic in the module wrapper, either, because the template builder doesn't allow nested Global Groups (which is, of course, a shortcoming on itself).