Add infinite-scroll to modal selector popups

* Extract infinite-scroll handling functionality to `InfiniteScroll`
  controller which simply triggers `scrollNext` events
* Explicitly create the controllers onExplicitly create the controllers
  `.sub-level` and `.selector-list` elements
* Refactor, slightly, the scrolling logic in selector lists
Merge pull request #1174 from reciprocity/feature/infinite-scroll-selectors

Add infinite-scroll to modal selector popups