Keep <noscript> when appropriate

[Treora]

This was previously issue #134 in webmemex-extension ("Images not in snapshots from Medium.com").

When the page was viewed with javascript disabled, we should keep <noscript> tags, and perhaps convert them into <div> tags, in order to make the snapshot correspond to what was rendered. See the corresponding comment in the source:

// If noscript content was not shown, we do not want it to show in the snapshot either. Also, we
// capture pages after scripts executed (presumably), so noscript content is likely undesired.
// TODO We should know whether noscript content was visible, and if so keep it in the doc.
// TODO Keep noscript content in fetched iframe docs, as scripts have not been executed there?
const noscripts = Array.from(doc.querySelectorAll('noscript'))
noscripts.forEach(element => element.parentNode.removeChild(element))