RenderedHTML always generates local_src, is slow

[neelnanda-io]

When I define a CircuitsVis component, it can take several seconds as it generates local_src, and especially running a bundle_source command. I often just want the cdn_src when I am not actively developing CircuitsVis (but using it elsewhere), but the components don't let me explicitly turn this off.

I may fix this myself at some point, but assigning to @alan-cooney in case there's a principled way of doing this - IIRC you used to have it another way and changed your mind?