Make the sidebar a web component

[karger]

Since NB places its html in the page itself, it is susceptible to influence from the CSS of the containing page. Every once in a while someone makes changes to the CSS on a page containing NB, and it breaks the NB sidebar. The introduction of web components and their shadow DOM provides a mechanism to shield a part of the page from the influence of the CSS on the containing page. I'm not sure we want to go all the way to creating a web component, but I think it should be pretty straightforward to use a declarative shadow DOM. Note that since the shadow DOM shields the CSS, you'll need to load the CSS for the NB sidebar into the element inside the shadow DOM, instead of into the containing page.