Completely replace perfect-scrollbar with SimpleBar.

perfect-scrollbar replaces both the appearance and the behavior of the
scrollbar, and its emulated behavior will never feel native on most
platforms.  SimpleBar customizes the appearance while preserving the
native behavior.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

static/js/scroll_util.js

@@ -34,6 +34,7 @@ exports.scroll_element_into_container = function (elem, container) {
     // this will be non-intrusive to users when they already have
     // the element visible.
 
+    container = ui.get_scroll_element(container);
     var elem_top = elem.position().top;
     var elem_bottom = elem_top + elem.innerHeight();