Fix drag handles CSS

In #2642810: UX improvements for nested paragraphs → we have developed a basic approach to improving the usability of nested paragraphs.

That issue left 2 outstanding problems with drag handles:

1) @Miro_dietiker found that "The draggable handle is not only modified for paragraphs itself. It is also changed for all multivalue fields inside a paragraph."

2) The plan was for drag handles to be vertically aligned with paragraph titles so that it is clear what is being dragged. However, they are actually pushed a little down, at least to me on Chrome Windows 10.