Optimize based on what is onscreen

[staab]

From #497

Use svelte-infinite-scroll. This component allows us to determine what is visible on screen, which might give us an opportunity to optimise performance by only requesting/computing information for the elements on-screen or nearby. I'm thinking mostly of NoteActions.svelte, this one is probably the most expensive.

Although it looks like svelte-infinite-scroll basically does the same thing that createScroller does. It would be nice to un-render stuff that is no longer visible on the screen (that has been scrolled past) without losing its place in the dom. We could probably do this pretty easily in a custom component.