Impulse: view clicked zettel in split view

[srid]

The non-static Impulse currently displays the folgezettel tree and allows the user to filter it by interactive search. Clicking a zettel will take you to the statically-generated HTML page.

However, I would like a way to view any zettel without leaving the Impulse page (especially as reloading Impulse is slow'ish due to JSON fetch). The zettel could open in a split pane (iframe?) whilst still keeping the filtered tree in sight.

Implementation approaches

• Easy way: use something like iframe to load the generated .html file.
• Render it afresh in GHCJS. This would allows us to experiment with alternate views like Display clicked zettel with a stacked multi-column layout #436

[srid]

As a stretch goal (this can happen in different PR), provide a way to reload cache.json if it gets updated, so the user doesn't have to Ctrl+R manually. Should be considered as part of a larger server revamp (cf. #200)