ui_init: Handle youtube play icon positioning via mouseenter handler.

Previously, we were experiencing a regression in the positioning of
the play icon for youtube previews, as such, this commit uses a
previously created `handle_video_preview_mouseenter` to ensure the
positioning is always correct.

This is an ugly way of doing things, because this could be handled
directly through CSS flexbox, however, it is an acceptable temporary
fix.

static/js/ui_init.js

@@ -335,7 +335,7 @@ export function initialize_kitchen_sink_stuff() {
     }
 
     $("#main_div").on("mouseenter", ".youtube-video a", function () {
-        $(this).addClass("fa fa-play");
+        handle_video_preview_mouseenter($(this));
     });
 
     $("#main_div").on("mouseleave", ".youtube-video a", function () {