Check if the split point suggested via T274539 is reasonable, and possibly move it. It needs to be where we will switch between the current template dialog and the new design. Prepare the existing code to be more flexible. Existing code can be moved around as long as we don’t change its behavior.
It’s important not to duplicate code, because we might be forked for as long as a year.
The scope of this task is mostly just making changes to the BookletLayout and TransclusionDialog.