Simplify embed resolution
Simplify embed resolution

Normally, the only referring widgets you would want to add are (fully
resolved) links, so this commit removes the resolution of referring
widgets, and instead adds a tailored function to include any such fully
resolved links.

jupyter-widgets/ipywidgets,jupyter-widgets/ipywidgets,ipython/ipywidgets,jupyter-widgets/ipywidgets,SylvainCorlay/ipywidgets,ipython/ipywidgets,ipython/ipywidgets,SylvainCorlay/ipywidgets,jupyter-widgets/ipywidgets,SylvainCorlay/ipywidgets,ipython/ipywidgets,SylvainCorlay/ipywidgets,ipython/ipywidgets