Replace slider implementation in Related Slider

[skleinfeldt]

Continuing the discussion started here: #143 (comment)

About mobile: On iOS, when it's active the scrollbar looks like it does on Safari desktop, but tiny and it only shows when scrolling is happening (that is, during swiping). The scrollbar itself doesn't seem to do anything on mobile. And there is no indication when looking at this part of the page that it is scrollable, no suggestion that there is more than one image.

Alec mentioned that if we want to use another JS slider implementation we'll have to research and pick one. Is it possible to find out from examining the page source what is used in the "Global Leaders Trust Six Feet Up" section of this page? https://sixfeetup.com/ That one looks and works like I'd expect on desktop and mobile. (Hoping it is not jQueryTools!)

[alecpm]

Six Feet Up is apparently using Owl carousel which looks like a pretty good option that doesn't require bringing in a ton of other stuff, though it's still fairly heavyweight and relies on a lot of JS configuration for styling.

I think we could add some simple JS back/forward buttons to scroll the scroller in a more intuitive way on desktop and solve most of the issues while still keeping the markup simple/semantic and CSS configurable, without bringing back a heavy JS library for the scroller.

[alecpm]

Owl is the first one (besides JQueryTools) I've seen that works well for scrolling a list of items responsively, so that's a big plus. The fact that all the other scroller implementations I saw were built around scrolling single elements (or grouped pages of elements), was why I went with a CSS based solution when upgrading the add-on.